SCLEROTINIA SNOW MOLD

Picture of snow mold:

field shot of infected wheat

Scientific name: Sclerotinia borealis

Hosts: Wheat, grasses (1)

Symptoms of Sclerotinia snow mold: In the spring, dead leaves and plants are gray and thready. Many black, irregular sclerotia, 2 - 4 mm in length, cover diseased tissues (1).

Lab diagnosis: Sclerotinia borealis can grow on media with low water potentials (-10 to -30 bars) and at temperatures down to -5 C (1).

Look-alike symptoms:

- Pink snow mold

- speckled snow mold (Typhula blight) (MB).

Favorable conditions: When damp, cold autumns are followed by deep snow cover that persists 5 months or more on slightly frozen ground (1).

Pattern in field: Sclerotinia snow mold usually occurs in isolated patches in fields (1).

Infestation levels:

Life cycle: Sclerotia germinate in the fall and produce cup-shaped apothecia lined with a fertile layer of asci. Ascospores are forcibly ejected and are wind-borne (1).

History in MT and US: Conditions favorable for the disease also exist in high-altitude regions of Canada, Europe, Scandanavia, Russia, and Japan (1).

Crop losses:

Cultural controls:

Resistant varieties:

Biological controls:

Chemical controls:

References: 1) Wiese, M.V. 1977. "Sclerotinia snow mold" Compendium of Wheat Diseases. APS Press, St. Paul, MN, pp. 34-35.
 
 
 
 

SEPTORIA LEAF AND GLUME BLOTCH

Pictures of septoria leaf and glume blotch:

comparison - between septoria and tan spot on wheat

glume blotch - on wheat head
 

Scientific name: Septoria nodorum

Hosts: Wheat (spring, winter, durum)

Symptoms:

leaf symptoms, #1

leaf symptoms, #2

stem symptoms

LEAF BLOTCH:

1) Leaf blotch appears first as light-green or yellow spots between the leaf veins. The spots spread rapidly to form light-brown irregular blotches (8).

2) Severely diseased plants have "fired" leaves (11).

3) Final diagnostic signs are tiny dark dots on the brown blotches. These dots are the sprore-producing chambers of the fungus.

4) Fruiting bodies on the wheat residue of the previous year can be viewed easily with a hand lens and feel like sandpaper to the touch (8)
 

GLUME BLOTCH:

Brown to tan spots with small, light brown speckles occur on the glumes. Often there will be lesions on the leaf sheaths. Symptoms appear on individual florets within heads. Generally, disease begins at the glume tip then advances downward to the glume base (1).

Lab diagnosis: If Septoria is not sporulating in the lesions, it may help to put leaves in humidity chamber for 24-48 h to induce pycnidia to develop (A. Shaeren, 7/94).

Look-alike symptoms of Septoria leaf and glume blotch:

1) Tan spot

a) the lesions occur earlier in crop's development than with Septoria;

b) no spots occur on the sheaths; and

c) fruiting bodies on stubble are larger than those produced by Septoria (i.e., they are visible with the naked eye whereas with Septoria, a hand lens is necessary to detect them)(A. Shaeren, 7/94).

3) Physiologic leaf spot (pinto spot)

4) Herbicide injury due to bipyridyliums (Avenge, Cyclone, Gramoxone, Gramoxone Extra, Reglone, Sweep)
 

Favorable conditions:

- Moist weather and continuously-cropped wheat favor the disease.

- Conservation tillage or systems that leave a high level of residue on the soil surface favor Septoria.

- Septoria can be severe regardless of plant density or fertility level. Early fall planting can increase infection levels (1).

- High phosporous levels and accompanying increased lodging have been shown to increase Septoria leaf blotch in the southeastern US (15)
 

Pattern in field: Symptoms often are fairly uniform within fields. However, symptoms may be more severe in chaff rows from previous wheat crops or in low-lying or wetter areas of fields.

Infestation levels: The presence of Septoria leaf blotch on lower leaves generally has little effect on yield. However, if the flag leaf becomes moderately to severely infected, yield reductions can occur.

Life cycle: The fungi causing Septoria leaf and glume blotches and tan spot do not exist as distinct races, although they do vary in their aggressiveness (12).

1) Both Septoria nodorum and S. tritici can be seedborne and survive on infected wheat stubble.

2) The fungus is spread by local air currents and splashing rain (11, 12).

3) Outbreaks are mostly limited to the fields in which the spores overwinter (12).

3) Infection can occur at any time during the season.

4) The organism requires at least 6 h of leaf wetness and the optimal temperature for infection is 68 - 80 F (APS Wheat compendium).

5) Septoria spores often get into the area between the leaf and the sheath where it is more humid even on hot, dry days. Black lesions may then arise in the sheath area (A. Shaeren, conversation, 7/94).

History in MT and US:

- In Montana, the Septoria complex is common on wheat grown under favorable conditions. However in most years, prevailing hot, dry weather prevents disease development past the yield-crucial post-boot stage thereby minimizing damage (16).

- Septoria leaf and glume blotches are most important on winter wheats in the central and southern states, from eastern Kansas through Indiana and into Georgia, and on spring wheats in the North Central states, such as Minnesota, North Dakota, and Montana (12).

Crop losses:

- The Septoria complex doesn't alter seed set much, but it does affect grain quality by causing shrivelled kernels and poor germination (1, 6)

- Up to 20% of the potential yield can be lost to foliar cereal pathogens such as Septoria leaf blotch in Montana given favorable environmental conditions (13).

- On a worldwide basis, the Septoria fungi destroy almost 2% of wheat annually (6) and losses of 50% or more can occur under favorable conditions (15).

Cultural controls: Crop rotation (2 years away from wheat is usually sufficient to eliminate most disease problems (1)). Crop rotation to crops other that grass or cereal, summer fallow, field burning, and plowing down of volunteer wehat and wheat stubble reduce the disease.

Resistant varieties: Fortuna spring wheat (ref. 2 or 3??); Roughrider and Norstar winter wheats have some resistance ( A. Shaeren). Duram wheats also have some resistance in that the septoria isolates that predominately affect durum wheat are not the same as those on hard red wheat (A. Shaeren, conversation, 7/94)

Below are comparative severities of Septoria leaf blotch in winter and spring wheat at different locations in MT. When making comparisons, take into account that it is best to compare single regions of Montana in different years. Favorable conditions or the disease organism itself will vary from one location to another. This can cause variation in ratings from year to year.

The following are relative severities of Septoria leaf blotch (Sidney, 1994; Creston, 1994; and Moccasin, 1993) and tan spot (Sidney, 1993) on WINTER WHEAT. L=Low level of disease; M=Moderate level of disease; H=High level of disease; VH=Very high level of disease; PLS=physiological leaf spot caused by the environment and not by a disease organism.
 

Location	Septoria (Sidney)	Septoria (Creston)	Septoria (Moccasin)	Tan Spot (Sidney)
AC Readymade	M-H	PLS	M	H
Agassiz	L-M	0	M	H
Arapaho	M	L	M-H	M-H
Blizzard	M	L-M	M-H	M-H
Centurk	M	L	VH	VH
Judith	M	0	H	M-H
Jules	M	H	0	-
Karl92	L-M	L	-	-
Kestrel	M-H	H	PLS	M
Lamar	M	M	M-H	M-H
Manning	L-M	M	L-M	M-H
Meridian	L	L	L	M-H
Neeley	M	L	M-H	M-H
Norstar	L	0	H	H
Promontory	-	H	M	H
Quantum542	M	PLS	-	-
Redwin	M	M	L-M	M-H
Rocky	M	M	H	H
Roughrider	M	H	H	H
S86-736	M	H	-	-
Tiber	L-M	M	L-M	M
Vista	L-M	0	-	-
Vona	H	0	H	M-H
Weston	M	0	M	H
Winalta	M-H	H	H	H
Winridge	L-M	M-H	L-M	H
Yuma	M-H	L	H	M-H

Biological controls: There is some evidence that if left in no-till continuous wheat, biocontrol organisms will build up and help control septoria (no evidence for this with tan spot) (A. Scharen).

Chemical controls: Foliar fungicides result in an average of 10-12% yield return in eastern ND. (7). However, due to economic reasons and the dependence of foliar diseases on weather conditions, fungicides are recommended in Montana only in the following situations:

1) The wheat crop has a good yield potential of at least 45 bu/acre.

2) Disease is present on the leaves below the flag leaf.

3) The weather is favorable for disease development (i.e., wet, humid, and relatively warm).

4) The 10-day forecast calls for continuation of above conditions and the chance of long dew periods.

5) The crop has enough soil moisture to achieve the yield potential (7).
 

If these conditions are present, the following are foliar fungicide alternatives:

1) Propaconizole, 41.8% (sold as Tilt by Ciba-Geigy)

2) Mancozeb (sold as Clean Crop Mancozeb 80WP, 80%; Clean Crop Mancozeb 4L, 37%; Dithane DF, 75%; Dithane F-45, 37%; Dithane M-45, 80%; Manzate 200 DF, 75%; Penncozeb, 80%; Penncozeb DF, 75%) (9). 1 pound of mancozeb costs approximately $2.75 (14).

GROWTH STAGE

PRODUCT^a RATE/ACRE OF APPLICATION INTERVALS
 

Tilt^b 4 oz. early flag leaf 1 applic. ony emergence allowed

Mancozebs^d 2 lb. Boot and 10 days 3 applic. after
 

^{cTilt also controls tan spot, powdery mildew, leaf rust, and stem rust (9,10).}

Seed treatment with carboxin-thiram combinations gives partial control of the seedborne phase of the disease.

References: 1) Lipps, P.E. 1988. Wheat disease control in Ohio. Dept. of Plant Path. Ohio State Univ. 2)? 3) Plant Disease Alert, KSU, 4/5/93 4) Pacific Northwest Plant Disease Control Handbook, 1994. 5) NDSU Crop and Pest Report, 5/27/94. 6) Wiese, M.V. 1977. "Septoria leaf and glume blotch" Compendium of Wheat Diseases. APS Press, St. Paul, MN, pp. 42-45. 7) NDSU Pest Report, "Foliar fungicides and wheat" June 11, 1993, No. 6. 8) NDSU Pest Report, Tan Spot and Septoria on wheat residue, May 27, 1994. 9) 1994 North Dakota Plant Disease Control Guide, "Wheat", NDSU Extension Service, p. 54-55. 10) NDSU Crop and Pest Report, "Fungicide use on small grains", June 17, 1994, Issue 7. 11) Guide to Herbicide Injury Symptoms in Small Grains, Second edition. 1992. Agri-Growth, Inc., Hollandale, MN, pp. 83-84. 12) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, pp. 54. 13) Riesselman, J. Foliar leaf spot complex on wheat. Montana Crop Health Report, May 12, 1989. 14) McMullen, M. Early season fungicide application on wheat. Crop and Pest Report, NDSU, May 20, 1994. 15) Leath, S., Sharen, A.L., Lund, R.E., and Dietz-Holmes, M. 1993. Factors associated with global occurences of Septoria nodorum blotch and Septoria tritici blotch of wheat. Plant Disease, Vol. 77, No. 12. 16) Watkins, J. Septoria leaf blotch of wheat. NebGuide, Cooperative Extension Service, University of Nebraska-Lincoln, Bulletin G84-701, April 1984.
 
 
 

SNOW ROT
 

Scientific name: Pythium spp. (snow rot)

Hosts:

Symptoms:

Look-alike symptoms:

Favorable conditions:

Pattern in field:

Infestation levels:

Life cycle: The Pythium species involved in snow rot differ from those that cause seedling damage and root rot. Snow rot results from motile spores of Pythium infect through natural openings (stomates) in wheat leaves and then digest away leaf tissues between the veins. Rotted leaves are commonly only fragments of remnant leaf tissue held together by the filamentous veins. The fungi produce their motile spores in ice water and cause the greatest damage to wheat trapped under snow or ice with water moving down the rows and over the leaves beneath it (1).

History in MT and US:

Crop losses:

Controls:

Cultural controls:

Resistant varieties:

Biological controls:

Chemical controls:

References: 1) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, p. 45.
 
 
 

SPECKLED LEAF BLOTCH
 

Scientific name: Septoria tritici

Hosts: Wheat

Symptoms of Speckled leaf blotch:

- Septoria leaf blotch

- Early symptoms include small, chlorotic flecks restricted by leaf veins.

- These spots usually develop longitudinally to form tan to reddish-brown, irregular-shaped blotches that are surrounded by a yellow halo.

- Advanced blotches develop light brown- to ash-colored centers with small black specks (pycnidia). The presence of these pycnidia are the most reliable symptom for distinquishing Septoria from other foliar diseases.

- Severely-infected leaves turn yellow and die prematurely.

- Another distinguishing characteristic of Septoria diseases is the development of symptoms on leaf sheaths and stems, especially at the nodes, and on the glume tips (3).

Look-alike symptoms:

- Septoria leaf and glume blotch: This disease and speckled leaf blotch are caused by different Septoria species. Speckled leaf blotch generally does not occur north of the Yellowstone River in Montana, whereas Septoria leaf and glume blotch is present throughout Montana. Cultural and chemical controls are similar although resistant varieties may vary between the two diseases.

- Tan spot: Tan spot does not occur in the leaf sheaths and at the nodes. Development of pycnidia does not occur in the lesion with tan spot.

Favorable conditions:

- High levels of wheat residue

- Continuous wheat production-

- Cool, wet weather in May and June.

- Wet periods fanging from 6 to 16 hours are required. (Dry periods prevent infections and halt lesion development).

- Temperatures of 60 to 70 F are optimal for infection and spread. However, infection can occur at temperatures as low as 40 F (3).

- In most years, prevailing hot, dry weather during the post-boot stage is unfavorable for further Septoria development during this yield-crucial stage (3).

- Conditions that are favorable for serious disease outbreaks also are favorable for high cereal yields (2).

Pattern in field: Symptoms often are fairly uniform within fields. However, symptoms may be more severe in chaff rows from previous wheat crops or in low-lying or wetter areas of fields.

Infestation levels: The presence of speckled leaf blotch on lower leaves generally has little effect on yield. However, if the flag leaf becomes moderately to severely infected, yield reductions can occur.

Life cycle: The fungus survives in wheat residue. Straw, seed, and volunteer wheat are primary inoculum sources. At least 6 hours of leaf wetness are necessary for infection to occur. Temperatures of 60 to 70 F are optimal for infection and spread, however, infection can occur at temperatures as low as 40 F (3). Previous year's residue coupled with favorable environmental conditions play important roles in disease development.

History in MT and US:

- In Montana, speckled leaf blotch is prevalent south of the Yellowstone River (wheat in the rest of the state is likely infected by another septoria disease: Septoria leaf blotch) (2)

- Speckled leaf blotch is the most common foliar wheat disease in Kansas as of 4/93 (1).

Crop losses:

- Since other foliar diseases such as tan spot and leaf rust often occur simultaneously with Septoria diseases, it is often difficult to measure damage due to each disease separately (3).

- Up to 20% of the potential yield can be lost to foliar cereal pathogens such as speckled leaf blotch (2).

- On a worldwide basis, the Septoria fungi destroy almost 2% of wheat annually (3) and losses of 50% or more can occur under favorable conditions (5).

Cultural controls:

The following cultural controls can minimize the impact of Speckled leaf blotch (as well as Septoria leaf blotch and tan spot):

- Crop rotation: 2 years away from wheat is usually sufficient to eliminate most disease problems (1).

- Summer fallow generally will reduce disease severity in subsequent wheat crops somewhat due to breakdown of infested wheat residue over time.

-Field burning and plowing under volunteer wheat and wheat stubble can greatly reduce Septoria diseases and tan spot by eliminating the overwintering structures of the fungus.

Resistant varieties:

Biological controls:

Chemical controls: Foliar fungicides result in an average of 10-12% yield return in eastern ND. (7). However, due to economic reasons and the dependence of foliar diseases on weather conditions, fungicides are recommended in Montana only in the following situations:

1) The wheat crop has a good yield potential of at least 45 bu/acre.

2) Disease is present on the leaves below the flag leaf.

3) The weather is favorable for disease development (i.e., wet, humid, and relatively warm).

4) The 10-day forecast calls for continuation of above conditions and the chance of long dew periods.

5) The crop has enough soil moisture to achieve the yield potential (7).

If these conditions are present, the following are foliar fungicide alternatives:

1) Propaconizole, 41.8% (sold as Tilt by Ciba-Geigy)

2) Mancozeb (sold as Clean Crop Mancozeb 80WP, 80%; Clean Crop Mancozeb 4L, 37%; Dithane DF, 75%; Dithane F-45, 37%; Dithane M-45, 80%; Manzate 200 DF, 75%; Penncozeb, 80%; Penncozeb DF, 75%) (9). 1 pound of mancozeb costs approximately $2.75 (4).

GROWTH STAGE

PRODUCT^a RATE/ACRE OF APPLICATION INTERVALS

Tilt^b 4 oz. early flag leaf 1 applic. ony emergence allowed

Mancozebs^d 2 lb. Boot and 10 days 3 applic. after

^{cTilt also controls tan spot, powdery mildew, leaf rust, and stem rust (9,10).}

Seed treatment with carboxin-thiram combinations gives partial control of the seedborne phase of the disease.

References: 1) Plant Disease Alert, Kansas State University, April 5, 1993 2) Riesselman, J. Foliar leaf spot complex on wheat. Montana Crop Health Report, May 12, 1989. 3) Watkins, J. Septoria leaf blotch of wheat. NebGuide, Cooperative Extension Service, University of Nebraska-Lincoln, Bulletin G84-701, April 1984. 4) McMullen, M. Early season fungicide application on wheat. Crop and Pest Report, NDSU, May 20, 1994. 5) Leath, S., Sharen, A.L., Lund, R.E., and Dietz-Holmes, M. 1993. Factors associated with global occurences of Septoria nodorum blotch and Septoria tritici blotch of wheat. Plant Disease, Vol. 77, No. 12.
 
 
 
 

SPOT BLOTCH

Scientific name: Cochliobolus spp.

Hosts:

Symptoms: Helminthosporium sativum produces distinct, elongate, brown-black lesions that rarely exceed 1 cm in length. Their color is diagnostic and due in part to fructifications of the fungus. The blotchy lesions contrast sharply with green or straw-colored leaf tissues even after leaf senescence.

Lab diagnosis: Plate on water agar, then transfer to V8 agar to induce sporulation.

Look-alike symptoms:

Favorable conditions:

Pattern in field:

Infestation levels:

Life cycle:

History in MT and US: Rare but found in MT in 1993.

Crop losses:

Cultural controls:

Resistant varieties:

Biological controls:

Chemical controls:

References: 1)Wiese, M.V. 1977. "Common dry land root and foot rot and associated leaf and seedling diseases" Compendium of Wheat Diseases. APS Press, St. Paul, MN, p. 52.
 
 

STEM RUST or BLACK RUST

Picture of stem rust:

on alternate host - barberry, aecia on underside of leaf

Scientific name: Puccinia graminis f.sp. tritici

Hosts: Wheat with alternate hosts being species of barberry (Berberis vulgaris, B. canadensis, and B. fendleri and some species of Mahonia. Sometimes minor infections occur on barley, rye, oat, and some grasses especially wild barley (Hordeum jubatum) and goatgrass (Aegilops spp.) but these are not considered important hosts (6).

Symptoms:

classic symptoms - on wheat stems in field

symptoms - on wheat leaves in field

Stem rust infects the stems and also can infect the leaves, sheath, glumes, and awns. Infection first appears as brownish-red elongate pustules (lesions) producing urediospores. As the plant matures, the fungus begins producing teliospores, which are black in color (8).

Look-alike symptoms:

Favorable conditions: The presence of spores and warm temperatures are the most crucial factors for disease to occur (7). Warm, wet weather and delayed crop maturity favor stem rust (6).

Pattern in field:

Infestation level:

Life cycle: The stem rust fungus has a higher temperature requirement than either stripe rust or leaf rust. Stem rust occurs on winter and spring wheats where the average daily spring the average daily highs during heading and grain fill are 70 F and above, the nights are warm, and leaves are kept wet by dew and rains (7). Stem rust has five spore stages in its complete life cycle and two hosts.
 

SPORE TYPES AND HOSTS:

Urediospores: Wheat and barley

Teliospores: Wheat and barley

Basidiospores: Wheat and barley

Pycnidiospores: Barberry

Aeciospores: Barberry

Although barberry is important to the fungus in producing new races (biotypes), it is not essential for the survival of the fungus. This is because the uredial stage is often called the repeating stage since it can continue to produce spores for as long as the plant is alive (8).

Barberry bushes along old fencerows or in backyards continue to serve as the main source of the occasional but rare outbreaks of stem rust in the Pacific Northwest states (7).

Stem rust can overwinter on an alternate host, unlike leaf or stripe rust. These latter diseases produce only urediospores (the red spore stage on leaves of wheat and are able to reinfect leaves of wheat only. The rust fungi are unique in their ability to move long distances by wind. This makes up for their inability to overwinter in fields in northern climates. Urediospores of the stem rust fungus have been found several thousand feet above the earth and can move hundreds of miles from a single source upwind (7, 8).

Conditions are too cold for stem rust to overwinter in Montana. Instead, it overwinters in Mexico and southern Texas and blows north by wind. Stem rust is favored in these warm climates, because the fungus requires high temperatures for building up its population before winter sets in. In warmer states, stem rust overwinters as mycelium of the uredial stage on early seed or volunteer wheat or grasses (?). Stem rust develops optimally near 20 C (6).

Rust fungi can survive the winter in the Great Plains states, but generally only if the leaves are protected from freezing by snow. In general, if the leaves of winter wheat can survive, the rust fungi in those leaves can survive. Winterkill, drought, and snow molds that eliminate the leaves of winter wheat also eliminate the rust fungi contained within them (7).

History in MT and US: Stem rust has been the cause of at least eight major rust epidemics in the U.S. since 1916 (8). Generally, stem rust is only economically important on a sporadic basis in the eastern third of Montana. In eastern Montana, 1987 was the worst stem rust year since the 1960's (1).

Crop losses: Reduces yield by up to 15-20% and reduces protein levels of grain.

Controls: Planting resistant varieties is the most effective control of stem rust. See Resistant varieties.

Cultural controls:

- Because of long-distance transport of spores, crop rotation and conservation tillage have no effect on rust fungi (7, 9).

- Since barberry is involved in the complete life cycle of stem rust, destruction of this shrub is a widely-used control method.

Resistant varieties: Use of resistant varieties is an excellent means of controlling leaf rust.

Winter wheat varieties:

RESISTANT: Agassiz, Arapahoe, Centurk, Judith, Lamar, RAM, Rocky, Roughrider, Seward, Vona, Yuma

MODERATELY RESISTANT: TAM 107

MODERATELY SUSCEPTIBLE: Blizzard, Norwin, Winalta

SUSCEPTIBLE: AC Readymade, Cree, Kestrel, Manning, Meridian, Neeley, Norstar, Promontory, Quantum 542, Redwin, Tiber, Weston, Winridge (4).

Spring wheat varieties:

RESISTANT: Amidon, Cutless, Fortuna, Glenman, Hi-line, Len, Lew, Newana, Olaf, Pondera, Stoa, Westbred Rambo, WestBred 926, WestBred 936

SUSCEPTIBLE: Thatcher (5).

An important distinguishing feature of the rust fungi is their occurrence as specialized races pathogenic on particular varieties of wheat. Resistance in wheats to these races can be complete (virtual immunity) but each new variety equiped with a single source of this kind of complete resistance tends to eventually select for a rust race that is able to overcome the plant's resistance to it. Virulent races can multiply to epidemic proportions even in a single season. Sometimes this occurs in the first season that a new resistant variety is grown, but more often it is in the 3rd to 5th season. For this reason, successful wheat varieties are usually bred with MULTIPLE sources of genetic resistance to the common races of rust fungi (7).

Biological controls:

Chemical controls: The following fungicides provide good protection against stem rust, but applications are only necessary when the potential for rust outbreaks is relatively high.

1) Tridemefon (Bayleton 50 WP) 2 - 4 oz ai/A.

Sprays should be applied in early heading but before "milk" stage when 10% of the tillers are affected. Use higher rates under severe disease pressure. Do not apply more than two applications or 8 oz ai/A. Do not apply within 21 days of harvest (3).

2) Propaconizol (Tilt) 4 fl oz/A in not less than 15 gal water (ground) or 5 gal water (air).

Apply at flag leaf emergence (Feeks growth stage 8). Do not apply after that growth stage. Do not apply more than once per growing season (3).
 

References: 1) Riesselman, J. Stem rust potential. Montana Crop Health Report, May 4, 1987. 2) 3) Pacific Northwest Plant Disease Control Handbook, 1994. 4) Winter wheat varieties. 1994. Montana State University Extension Service, Bulletin 1098. 5) Spring wheat varieties. 1994. Montana State University Extension Service, Bulletin 1093. 6) Wiese, M.V. 1977. "Stem rust" Compendium of Wheat Diseases. APS Press, St. Paul, MN, p. 38. 7) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, p. 51-53. 8) Watkins, J.E. 1985. "Stem rust of wheat" NebGuide. Cooperative Extension Service, U. of Nebraska, G73-68. 9) Lamey H.A and McMullen, M.P. 1993. "Crop rotations for managing plant diseases" North Dakota Extension Service. Bulletin PP-705.
 
 
 
 

STRIPE RUST

Scientific name: Puccinia striiformis

Hosts: Wheat, barley, native grasses

Symptoms of stripe rust:

leaf symptoms - on a field plant

field symptoms - in Switzerland

- Citron-yellow uredia (spore masses) appear in long stripes on the leaves and rarely on the stems and heads.

- As the crop matures, black spores (teliospores) are produced in stripes which are covered by the epidermis.

Look-alike symptoms: Leaf rust

Favorable conditions: This rust is favored by cool summers, mild winters, and prolonged cool, wet springs.

Pattern in field:

Infestation levels:

Life cycle:

- Stripe rust has lower temperature requirements than either leaf rust or stem rust. The fungus infects well in areas such as western Montana where temperatures during tillering and heading range between 40 and 60 F.

- The fungus survives as dormant mycelia on winter wheat and wild grasses in areas of high snow cover. It overwinters on the susceptible plant. It may take several years to build up populations.

- Conditions for stripe rust in western Montana typically are suitable for overwintering and oversummering of stripe rust year round.

- Stripe rust does not occur in the southeast even though temperatures are ideal for overwintering on winter wheat. Probably, this is because it is too hot there for the spores to oversummer and there is no external source of spores to be blown into the area.

History in MT and US: Stripe rust occurs mainly in the Pacific Northwest states including western Montana and northern California, where temperatures during tillering and heading range between 40 and 60 F. Conditions for stripe rust in western Montana typically are suitable for overwintering and oversummering of stripe rust year round. Stripe rust does not occur in the southeastern US even though temperatures are ideal there for overwintering on winter wheat. Probably, this is because it is too hot there for the spores to oversummer and there is no external source of spores to be blown into the area (5).

Crop losses:

Cultural controls: Because of long-distance transport of spores, crop rotation and conservation tillage have no effect on rust fungi (5, 6).

Resistant varieties: Use of resistant varieties is an excellent means of controlling leaf rust. An important distinguishing feature of the rust fungi is their occurrence as specialized races pathogenic on particular varieties of wheat. Resistance in wheats to these races can be complete (virtual immunity) but each new variety equiped with a single source of this kind of complete resistance tends to eventually select for a rust race that is able to overcome the plant's resistance to it. Virulent races can multiply to epidemic proportions even in a single season. Sometimes this occurs in the first season that a new resistant variety is grown, but more often it is in the 3rd to 5th season. For this reason, successful wheat varieties are usually bred with MULTIPLE sources of genetic resistance to the common races of rust fungi (5).

Redwin and Tiber are especially susceptible; Windridge is more resistant (ref?).

Hard red winter wheat:

RESISTANT: Blizzard, Manning, Promontory, RAM, Weston, Winridge MODERATELY RESISTANT: Meridian, Norwin

MODERATELY SUSCEPTIBLE: Seward, Winalta

SUSCEPTIBLE: Centurk, Kestrel, Norstar, Quantum 542, Rocky, Roughrider, Vona.

VERY SUSCEPTIBLE: AC Readymade, Arapahoe, Cree, Judith, Lamar, Redwin, TAM 107, Tiber, Yuma (3).

Hard red spring wheat:

RESISTANT Amidon, Stoa, Len, Olaf, Westbred Express, Westbred 926, Westbred 936.

MODERATELY RESISTANT: Fortuna, Lew, Thatcher, Glenman, Newana, Pondera.

MODERATELY SUSCEPTIBLE: Cutless, Hi-line.

SUSCEPTIBLE: Westbred Rambo (4).
 
 

Soft white spring wheat varieties:

RESISTANT: Owens, Penawawa.

Chemical controls:

1) Seed treatment with Baytan 30F reduces seedling infections.

2) On infected plants, spray tridimefon (Bayleton) 2 to 4 oz ai/A - one or two applications. When using one application, apply 4 oz ai/A rate immediately after flag leaf emergence. When using two applications, apply 2 - 4 oz ai/A, depending on disease severity, at flag leaf emergence and just before flowering. Do not apply within 21 days of harvest. Do not apply more than 8 oz ai/A.

3) Or spray propaconizol (Tilt) 4 fl ozA in not less than 15 gal water by ground or 5 gal water by air. Apply at flag leaf emergence (Feeks growth stage 8). Do not apply after that growth stage. Do not apply more than once per growing season.

References: 1) 2) Pacific Northwest Plant Disease Control Handbook, 1994. 3) Winter wheat varities. 1994. MSU Extension Service Bulletin 1098. 4) Spring wheat varities. 1994. MSU Extension Service Bulletin 1093. 5) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, p. 51-53. 6) Lamey H.A and McMullen, M.P. 1993. "Crop rotations for managing plant diseases" North Dakota Extension Service. Bulletin PP-705.
 
 

TAN SPOT

Pictures of tan spot:

pseudothecia - on wheat residue

Pyrenophora trichostroma - ascospores, wheat

Pyrenophora - conidia from wheat
 

Scientific name: Pyrenophora tritici-repens (Died.) Drechs. (anamorph=Drechslera tritici-repentis (Died.) Shoemaker)

Hosts: Wheat and some wild grasses. Barley and oats are susceptible under favorable environmental conditions although losses to these crops will be minimal (17,18).

Symptoms of tan spot:

leaf symptoms

- Tan spot causes elliptical or diamond-shaped tan lesions.

- Spots may have a yellow border and a small dark brown center. A variety-specific toxin causes the necrotic and chlorotic symptoms (12).

- Older leaves are more susceptible to tan spot than are younger ones (12).

- "Firing" of the leaves may occur.

- RED SMUDGE symptoms: a kernel infection caused by the same fungus that causes tan spot causes individual spikelets to look prematurely bleached (5).
 

Lab diagnosis: To isolate Pyrenophora tritici-repentis from diseased leaf tissue:

1) Surface sterilize symptomatic tissue in 10% sodium hypochlorite for 1-2 min.

2) Place on potato dextrose agar (PDA) plates.

3) Transfer colonies suspected of being Pyrenophora tritici-repentis to V-8 agar plates.

4) Place at room temperature under a 24 hr florescent light cycle for 3-4 days until colonies are 1/2 the diameter of the plate.

5) At this time the light cycle should be modified to a 12-hr on-off cycle (20).

Look-alike symptoms:

- Septoria leaf blotch

- Physiologic leaf spot (Pinto spot)

- Herbicide injury due to bipyridyliums (Avenge, Cyclone, Gramoxone, Gramoxone Extra, Reglone, Sweep)
 

Favorable conditions:

- Continuously-cropped wheat with high residue levels

- Cool, wet weather

- Continously-cropped wheat fields under no-till. Tan spot generally is more severe no-till fields due to survival of tan spot on plant residue. Conditions are even more favorable if wheat is under continuous production under no-tillage (3).

- Dry summer fallow followed by wet conditions during the wheat seedling phase can promote severe tan spot (19). More information (TS-FC1)

- Nitrogen fertilizers (NH₄SO₄ and CaNO₃) appear to reduce the disease indirectly through the delay of natural leaf senecence. However, they don't have a direct effect on the tan spot fungus (12).

Pattern in field:

- From a distance, the field will have a yellow cast (4).

- Tan spot is not limited to any one part of the field (10).

Infestation levels: Tan spot generally does not cause significant yield loss in Montana unless conditions allow the disease to develop on the upper leaves and especially the flag leaf (10). If disease is severe on the flag leaf, carbohydrates and nutrients going to the heads going towards grain filling will be reduced substantially. If the tan spot fungus is severe on flag leaves, it may move onto the heads and cause black point and pink smudge symptoms. This can result in yield and quality reductions (14).

Monitoring information:

SCOUTING can help estimate disease potential.

1) If several hundred or more pseudothecia per square yard on straw are counted, disease pressure will be moderate to high. If only a few are counted, pressure will be low.

2) Also, look for primary infections on young plants. If most leaves have at least one tan spot lesion, disease pressure may be high.
 

Life cycle:

- Spores are air-borne and also spread by splashing rain (10). Ascospores, conidia, and hyphal fragments all serve as primary inoculum. These propagules can be detected in the air from June until October in ND. They are most abundant during the morning hours of July and August. Wheat infections are most severe in locations close to infected wheat residue.

- The tan spot fungus also can be seedborne (13). More information (TS-LC1)

- The fungus infects best at temperatures in the range of 55 - 65 F. (11).

- A 6- to 48-hour wetting period is required for infection which can occur throughout the growing season (2).

- Older leaves are more susceptible to tan spot than are younger ones (12).

- The fungus survives as mycelia or pseudothecia on infected straw and stubble, on wild grasses and on rare occasions on seed (1, 12). Survival is through pseudothecia. These are small (about 1/64 inch), pear-shaped black structures that can be seen easily with the naked eye. Generally, inoculum travels only a few inches from infested straw so the amount of primary inoculum on wheat leaves in spring is related to the number of pseudothecia on straw in the immediate vicinity (1).

- After senescence of the plant begins, the tan spot fungus grows rapidly through the dying tissue. Even though the fungus is considered a weak saprophyte, it does have a competitive advantage over other fungi since it is able to gain a "foothold" on host tissue before other organisms move in. Perithecia form if enough moisture is available. The resistant perithecia that form subsequently can persist for long periods of time on the stubble (17).

History in MT and US: As the practice of retaining wheat residue on the soil surface has become more important in the past 15 years, tan spot has increased in incidence and severity in the Northern Great Plains. Tan spot is important on winter wheat in the central Great Plains and on spring wheats in the northern Great Plains and the Great Lakes area (11). Disease development is very dependent on weather conditions and cropping history. Field surveys of wheat leaf and head diseases from 1987 - 1991 in over 300 fields in North Dakota revealed that tan spot was the most prevalent of these diseases in all but one year (wheat streak mosaic was more prevalent in 1988). Tan spot incidence ranged from 6% of all fields in 1988 to 96% of all fields in 1991. Tan spot was more important in eastern ND counties where rainfall is more abundant than in western counties (16).

Crop losses:

- Up to 20% of the potential yield can be lost to foliar cereal pathogens such as tan spot in Montana given favorable environmental conditions (21).

- In Kansas, tan spot has caused average annual losses of $17 million dollars in the past 10 years (12).

Controls: Cultural practices that allow for rapid breakdown of tan spot-infected wheat residue (i.e. rotations, burning, deep residue incorporation) provide the best tan spot controls. In high-yield potential wheat under conditions favorable to disease development, foliar fungicides provide good control of tan spot.

Cultural controls:

- No-till or minimum till farming practices favor tan spot since the pathogen overwinters on wheat residue. The following cultural controls can minimize the impact of tan spot (as well as Septoria leaf blotch):

- Crop rotation (2 years away from wheat is usually sufficient to eliminate most disease problems (1)).

- Infection of barley will occur under favorable environmental conditions although losses to this crop will be minimal. However, the tan spot fungus can colonize senescing leaves and form pseudothecia on barley stubble. Therefore in some years, rotations to barley may not cause enough reduction in tan spot inoculum to significantly reduce the disease in the following wheat crop (17).

- Mulched stubble in direct contact with moist soil often favors rapid development of fruiting bodies pseudothecia while undisturbed, upright stubble often is free of fruiting bodies (18).

- Summer fallow generally will reduce tan spot severity in subsequent wheat crops due to breakdown of infested wheat residue over time.

-Field burning and plowing under of volunteer wheat and wheat stubble greatly reduce tan spot (and Septoria) by eliminating the overwintering structures of the fungus.

Resistant varieties: Commercial wheat varieties do differ in tolerance to tan spot as the tables below exemplify. However, varieties with high tolerance (low disease severity) may support just as much disease carryover into the following year as those with low tolerance to tan spot (18).

The following is a listing of relative severities of tan spot on winter wheat varieties. The field evaluations were conducted in Sidney, MT in 1993. L=Low level of disease; M=Moderate level of disease; H=High level of disease; VH=Very high level of disease.
 
 
 

Location	Disease Level		Location	Disease Level
AC Readymade	H		Norstar	H
Agassiz	H		Norwin	H
Arapaho	M-H		Promontory	H
Archer	-		Quantum542	-
Bighorn	-		Redwin	M-H
Blizzard	M-H		Rocky	H
Centurk	VH		Roughrider	H
Hawk	-		S86-736	-
Judith	M-H		Tiber	M
Jules	-		Vista	-
Karl92	-		Vona	M-H
Kestrel	M		Weston	H
Lamar	M-H		Winalta	H
Manning	M-H		Winridge	H
Meridian	M-H		Yuma	M-H
Neeley	M-H

Biological controls: In laboratory experiments, Luz (Brazil) found three bacteria that when used as seed treatments, showed equal or superior performance to iprodione + thiram fungicide seed treatments (15). More information (TS-BC1) Note: This work has not been tested in the field!

Chemical controls: Foliar fungicides result in an average of 10-12% yield return in eastern ND. (7). However, due to economic reasons and the dependence of foliar diseases on weather conditions, fungicides are recommended in Montana only in the following situations:

1) The wheat crop has a good yield potential of at least 50 bu/acre.

2) Disease is present on the leaves below the flag leaf.

3) The weather is favorable for disease development (i.e., wet, humid, and relatively warm).

4) The 10-day forecast calls for continuation of above conditions and the chance of long dew periods.

5) The crop has enough soil moisture to achieve the yield potential (7).

If these conditions are present, the following are foliar fungicide alternatives:

1) Propaconizole, 41.8% (qold as Tilt by Ciba-Geigy)

2) Mancozeb (sold as Clean Crop Mancozeb 80WP, 80%; Clean Crop Mancozeb 4L, 37%; Dithane DF, 75%; Dithane F-45, 37%; Dithane M-45, 80%; Manzate 200 DF, 75%; Penncozeb, 80%; Penncozeb DF, 75%) (7). 2 lb of mancozeb costs approximately $5.50 (9).

GROWTH STAGE

PRODUCT^a RATE/ACRE OF APPLICATION INTERVALS
 

Tilt^b 4 oz. early flag leaf 1 applic. ony emergence allowed

Mancozebs^d 2 lb. Boot and 10 days 3 applic. after

^{cTilt also controls Septoria leaf blotch, powdery mildew, leaf rust, and stem rust (7,8). Application of 2 oz./acre Tilt at the 5-leaf stage gave a good yield response in 2 ND locations in 1993. However, the label does not allow for more than one application per season so a second application cannot be made at flag leaf emergence (8).}
 

Foliar fungicides labelled for use on wheat include: Tilt or tank mix of Bayleton plus mancozeb. But neither treatment will achieve 100% control with just one application. Tilt: approx. $18/acre in 1993; not recommended except on irrigated wheat with more than 50 bu/acre yield potential. Next year, grow a resistant variety if you plant into minimum tillage wheat. Dithane 2 lb/acre, 26 days before harvest or just before heading, high yield potential fields only (J. Riesselman, conversation, 8/93)

References:

1) Plant Disease Alert, KSU, 4/5/93

2)

3) Lipps, P.E. 1988. Wheat disease control in Ohio. Dept. of Plant Path. Ohio State Univ. 2) Wiese, M.V. 1977. "Yellow leaf spot or tan spot" Compendium of Wheat Diseases. APS Press, St. Paul, MN, p. 41.

4) NDSU Pest Report "Tan spot of wheat" May 29, 1992, No. 5.

5) NDSU Pest Report "Tan spot of wheat" July 24, 1992, No. 13.

6) NDSU Pest Report, "Foliar fungicides and wheat" June 11, 1993, No. 6.

7) 1994 North Dakota Plant Disease Control Guide, "Wheat", NDSU Extension Service, p. 54-55.

8) NDSU Crop and Pest Report, "Fungicide use on small grains", June 17, 1994, Issue.

9) NDSU Crop and Pest Report, July 15, 1994, Issue 11.

10) Guide to Herbicide Injury Symptoms in Small Grains, Second edition. 1992. Agri-Growth, Inc., Hollandale, MN, pp. 85-86.

11) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, pp. 54.

12) Bockus, W.W. and Davis, M.A. 1994. Effect of nitrogen fertilizers on severity of tan spot of winter wheat. Plant Disease, Vol. 77 No. 5, pp. 508-510.

13) Schilder, A.M.C., Bergstrom, G.C. 1992. Infection of wheat seed by and seed transmission of Pyrenophora tritici-repens. Proceedings of the Second International Tan Spot Workshop, Fargo, ND.

14) Francl, L.J. and Jordahl, J.G. 1992. Spike inoculation of durum wheat with Pyrenophora tritici-repens and seed symptomology. Proceedings of the Second International Tan Spot Workshop, Fargo, ND.

15) Luz, W.C. 1992. Seed microbiolozation to control Dreschlera tritici-repentis. Proceedings of the Second International Tan Spot Workshop, Fargo, ND.

16) McMullen, M.P. 1992. Tan spot and five years of wheat disease survey. Proceedings of the Second International Tan Spot Workshop, Fargo, ND.

17) Summerell, B.A. and Burgess, L.W. 1988. Saprophytic colonization of wheat and barley by Pyrenophora tritici-repentis in the field. Trans. Br. Mycol. Soc., v. 90 (4), pp. 551-556.

18) Summerell, B.A. and Burgess, L.W. 1988. Factors influencing production of pseudothecia by Pyrenophora tritici-repentis. Trans. Br. Mycol. Soc. v.90 (4), pp. 557-562. 19) Summerell, B.A. and Burgess, L.W. 1989. Factors influencing survival of Pyrenophora tritici-repentis: stubble management. Mycol. Res., v.90 (1), pp. 38-40.

20) Raymond, P.J., Bockus, W.W., and Norman, B.L. 1985. Tan spot of winter wheat: procedures to determine host response. Phytopathology, v. 75, pp. 686-690.

21) Riesselman, J. Foliar leaf spot complex on wheat. Montana Crop Health Report, May 12, 1989.
 
 
 
 

TYPHULA BLIGHT OR SPECKLED SNOW MOLD
 

Picture of typhula blight:

on winter wheat - field in Bozeman, MT

Scientific name: Typhula spp.

Hosts: Cereals, grasses, legumes, sugar beets, and broad-leaved weeds (2).

Symptoms of speckled snow mold:

- A felty, white fungus mat forms over the plants as the snow disappears (1).

- Plants are easily shattered (2).

- Signs of snow mold disappear as the temperature rises and sunlight increases.

- Numerous black or brown, pin-head sized sclerotia (0.5 to 3 mm in diameter) on the surface of dead leaves (1,2).

Look-alike symptoms:

- Typhula blight usually has a greater association with snow cover than pink snow mold.

- winterkill

Favorable conditions: Snow molds usually occur where wheat has been under snow cover for at least 90 days. Conditions are most favorable when the snow first fell on unfrozen soil (3). Winter wheat in areas of high snowfall, at relatively high altitudes in certain intermountain valleys (Gallatin, Dillon, etc??)(1)

Pattern in field:

Infestation levels:

Life cycle: Typula spp. are poor saprophytes and survive as parasites or sclerotia. Scerotia germinate in the fall in response to moisture. They then form upright, club-shaped basidiocarps. Winter wheat is infected either by basidiospores or infectious hyphae from soil-borne sclerotia. In the US, basidiospores are most frequent in mid-November and maintain viability for 60 days (2).

History in MT and US:

Crop losses:

Cultural controls: In Idaho, rotation to legumes has reduced incidence in the following winter wheat crop. - Charcoal as coal dust applied on snow cover to speed snow melt is helpful (1).
 

Resistant varieties:

Biological controls:

Chemical controls:

References: 1) Pacific Northwest Plant Disease Control Handbook, 1990. 2) Wiese, M.V. 1977. "Speckled snow mold or typhula blight" Compendium of Wheat Diseases. APS Press, St. Paul, MN, p. 33. 3) Mathre, D. Snow mold of winter wheat. Montana Crop Health Report, April 11, 1988.