darkened
nodes - in mature field plants
Scientific name: Colletotricum spp.
Hosts: Sorghum and rye are the most important hosts, but barley, oats, corn, wheat, and about 20 genera of temperate climate grasses also are susceptible (1).
Symptoms: Lesions (elliptical in shape and 1-2 cm long) occur above and below ground. In early stages, lesions are water-soaked; later they become bleached and necrotic. Lesions normally are confined to the lower stem and acervuli generally do not develop until the plant matures (1).
Look-alike symptoms: Lesions resemble those of eyespot and sharp eyespot until dark acervuli appear in them (1).
Lab diagnosis: Acervuli contain diagnostic, dark spines visible under low magnification. Colonies on potato dextrose agar (PDA) are gray and felt-like. Conidia and appressoria are numerous when cultures are well aerated and sclerotia sometimes form. Appressoria are diagnostic, tawny-brown, prominent and terminal on thickened hyphae. They assume irregular shapes, average 11.2x15.6 um and have a single germ pore (1).
Favorable conditions: Continuously-cropped wheat, alkaline soil, and the presence of alternative grassy hosts promote build-up of inoculum. Wet weather favors infection. Optimum conditions for infections include presence of susceptible cultivars, 25 C, and free moisture (1).
Pattern in field:
Infestation levels:
Life cycle: Colletotricum graminicola survives as mycelia and conidia on numerous cereal and wild grass hosts and on plant residue. The primary inoculum is conidia dispersed by wind or water. These conidia germinate when in contact with water films. The appressoria can penetrate the plant directly. The soil-borne nature of the fungus allows for the prominence of root, crown, and basal stem infections.
History in MT and US: Anthracnose rarely is a problem in MT. The disease was more important from 1910 until 1940 worldwide. It does occur worldwide now, but is damaging only in isolated areas, especially in nutritionally- or other-stressed crops (1).
Crop losses: Severe infections of culms or crowns reduce plant vigor and promote head blighting, lodging, and shrivelled grain (1).
Cultural controls: Rotations with legumes or other non-hosts limit the build-up of inoculum. Clean cultivation eliminates alternate hosts. Adequate fertility reduces disease severity (1).
Resistant varieties: Some varieties show high anthracnose resistance, but the use of these is not widespread (1).
Biological controls:
Chemical controls: Seed treatments can help minimize anthracnose (1).
References: 1) Wiese, M.V. 1977. "Anthracnose" Compendium
of Wheat Diseases. APS Press, St. Paul, MN, pp. 27-28.
Scientific name: Ascochyta tritici
Hosts: Wheat
Symptoms: Early lesions are distinct, chlorotic, oval, or round, and 1-5 mm in diameter. Advanced lesions are diffuse and gray-brown internally. Pycnidia sometimes form and appear as black dots within the necrotic lesions. The pycnidia are submerged in the host tissue except for a papillate projection (1).
Look-alike symptoms: Advanced lesions resemble those of Septoria leaf blotch (Septoria nodorum).
Favorable conditions: High humidity, dense foliage, and leaves in contact with soil favor the Ascochyta leaf spot.
Pattern in field:
Infestation levels:
Life cycle: The fungus survives as mycelia and pycnidia in host residue. The primary inoculum is unknown, but pycnidiospores are liberated during wet weather and may serve in this role.
History in MT and US: Present in MT, North America, Europe, and Japan (1).
Crop losses: In general, the disease is of minor economic importance (1).
Controls: Specific controls for Ascochyta leaf spot are not prescribed, but controls for Septoria leaf blotch should be effective.
Cultural controls:
Resistant varieties:
Biological controls:
Chemical controls:
References: 1) Wiese, M.V. 1977. "Ascochyta leaf spot" Compendium
of Wheat Diseases. APS Press, St. Paul, MN, pp. 25-26.
Pictures of cephalosporium stripe:
darkened
nodes - in mature field plants
Stevens
wheat with 100% cephalosporium
Scientific name: Cephalosporium tritici Nisikado & Ikata.
Hosts: Winter wheat is the main economic host. Dryland winter wheat is much more severely affected that irrigated winter wheat (4) but other winter cereals (oats, barley, rye, triticale) and cheatgrass, Poa sp., Bromus sp., and Dactylis sp. also are susceptible on rare occasions. (Spring grains escape serious infection by avoiding winter frost heaving)(5).
Symptoms of Cephalosporium stripe:
symptoms
on winter wheat plant - in field, #1
symptoms
on winter wheat plant - in field, #2
- During jointing and heading, dwarfed plants and continuous yellow stripes on leaves become apparent. Normally, only one or two stripes and discolored veins are present per leaf.
- The continuity of stripes, and one or more darkened veins within them is diagnostic (10).
- After plants are mature, darkened stripes on stem of senescing plants run continuously along vascular tissue of entire plant.
- The nodes on the stems may be darkened.
- At heading and during grain filling, the awns of diseased plants tend to turn outward and the heads turn white (10).
- Heads are white and either empty or filled with shrivelled kernels (3,4).
- Plants in severely-infected fields may appear mature early (10).
- Some plants may only be half the height of healthy plants and develop smaller heads with normal-sized grain.
Lab diagnosis:
- For lab diagnosis, sterilize symptomatic leaf and/or stem tissue in 10% bleach for 30-90 sec.
- Cut off ends of tissue and plate on corn meal agar (CMA) or Cephalosporium selective media.
- Colonies are white and "ropey" in texture (Don Mathre, Martha Bamford, MSU).
Look-alike symptoms: Severely-infected fields have a yellowish cast that can be confused with yellowing caused by cool, wet soils and slow nitrogen uptake, leaf spot diseases (10).- Wheat streak mosaic. In some plants infected with Cephalosporium stripe, leaves may develop a mild mosaic-like pattern early in the season that can be confused with wheat streak mosaic. These leaves usually are short-lived and do not develop stripes (10). White head symptoms also can be caused by: take-all, fusarium foot rot, fusarium scab, wheat stem maggot, or Hessian fly.
Favorable conditions:
- Continously-cropped dryland winter wheat. (Spring wheat generally is not infected.)
- Early fall seeding is conducive to the disease since it allows more root growth before winter.
- FREEZE-THAW CYCLES break roots and allow the fungus to enter.
- Fluctuating winter temperatures (10).
- Early spring and prolonged cool, moist conditions in the spring (10).
- Seeding of susceptible varieties (10).
- Wet, poorly-drained soils (4).
- High wheat residue levels on soil (10)
- Cephalosporium often is more severe in fields with low pH (3). With the increased use of ammonium-based nitrogen fertilizer, there has been a gradual drop in soil pH in many areas (7).
- Cephalosporium stripe affects winter wheat in higher-rainfall areas managed for maximum yield (7).
- A winter wheat-fallow-winter wheat cropping system can cause severe Cephalosporium stripe since the fungus can remain viable during the 12-18 months between crops (11).
- Grassy weeds such as cheatgrass (Bromus tectorum) can serve as hosts for Cephalosporium and negate rotational effects (11).
- Cephalosporium rarely is seed-borne. However, poorly-cleaned seed can carry infected chaff into previously disease-free fields (11).
- Although many of the conditions that favor Cephalosporium stripe also favor take-all, the presence of one disease has not been shown to increase the severity of the other (synergism). Instead, there may a slight antagonism between the fungi causing these two diseases (8). More information (CS-FC1)
Pattern in field: Distribution of infected plants generally is random throughout the field (5, 10). Symptoms may be more frequent where more frost heaving occurred during the winter (MB) or in lower, wetter areas of the field (5).
Infestation levels:
- Disease symptoms can occur at any stage, but are more obvious during jointing and heading (10).
- In experiments conducted in the 1970's, Mathre et al found that each crop of early-seeded winter wheat seemed to allow a 2- to 3-fold increase in Cephalosporium stripe. For example, a field with an initial infection of 1% would show an increase to 3% the next year and 9% the 3rd year. In the 5th cropping year, infection percentage would be more than 70% (11).
Life cycle:
-The fungus is soil-borne and is the only true vascular pathogen in wheat. The fungus invades and becomes established in the vascular system of the roots. Germinated spores enter plant roots through wounds caused by freeze injury, mechanical tearing due to soil heaving, root-feeding inseccts or by other kinds of injuries.
- When the plant greens up in the spring, the fungus moves upward through the water transport system into leaves and elongating tillers. It impedes transport of water and nutrients through nodes, and leaf veins (5). This causes the striping symptoms and death of the plant (10).
- Cephalosporium overwinters in diseased crop residue. The survival of the fungus is reduced if the crop residue is destroyed. The fungus cannot survive by itself in soil for more than a few months (11).
-Cephalosporium produces spores for dispersal in water during cool, wet periods in the fall and early spring. The spores are very tiny and percolate downward with water into the soil until they come into contact with roots. Infection of new plants occurs through wounds in the root system caused by frost heaving, freeze damage, or cultivation (7).
- Cephalosporium can live as long as 2 years in straw in or on the soil. For this reason, it is necessary for rotations out of winter wheat to last at least 2 successive years for control of this disease.
- In more acidic soils, the fungus can infect wheat through unwounded roots. It also can survive longer in wheat straws in acid soils than in neutral or alkaline soils (7).
- Cephalosporium rarely is seed-borne. However, poorly-cleaned seed can carry infected chaff into previously disease-free fields (11).
History in MT and US: Cephalosporium stripe was first recognized in Japan in 1930. The disease was not present in the US until the 1950's when it was found in Washington (5, 10). In Montana, Cephalosporium stripe was first reported in 1958 in Big Horn, Cascade, and Gallatin counties. Now it is found in all areas of MT where winter wheat is grown (6). Cephalosporium stripe occurs in the winter wheat-growing areas of the inland Pacific Northwest, central and western Montana, and into central Kansas (7).
Crop losses:
- Cephalosporium stripe results in heads that are either empty or filled with shrivelled kernels.
- Under severe disease pressure, losses can approach 100%. Infected plants generally produce 70% less grain than do healthy plants (11).
- In 1976, 30-100% yield reductions were reported and Choteau Co. lost $10 million to Cephalosporium (11).
- In 1993, 100% losses occurred in isolated fields near Great Falls.
- In 1991, some Montana producers reported estimated yield losses of 25 bu/acre in areas where winter wheat had been grown for several successive production years (9).
Controls: Cephalosporium stripe generally can be controlled by good cultural practices.
Cultural controls:
1) Rotation:
Insert spring grains and crops into a 3-year rotation to break the disease cycle. Rotations to spring wheat, barley, and/or oats are equally effective in reducing Cephalosporium stripe. However, rotational systems where there is no fallow and winter wheat is early seeded generally do not reduce the disease (11).
2) Seeding:
Delay planting of winter wheat to when soil temperature is less than 55 F (3 inches deep at 1 pm) and reduce preplant nitrogen application. This limits wheat plant size during winter and reduces root breakage and freezing. Late planting also minimizes damage due to wheat streak mosaic, root and crown rot, and Hessian fly (10).
3) Tillage:
Reduce residue through tillage or burning if possible (4). A Kansas study reported that burning of the straw followed by disking was the most effective method of minimizing the disease in the next season (10). However, this often is not practical in Montana due to conservation requirements.
Resistant varieties: Bob Johnston and Don Mathre have released germplasm from 3 tolerant varieties (7/93). There are no highly resistant varieties available.
Biological controls:
Chemical controls:
References:
1) Montana Plant Health Report, May 15, 1980
2) Montana Plant Health Report, August, 13, 1980.
3) Lipps, P.E. 1988. Wheat disease control in Ohio. Dept. of Plant Path. Ohio State Univ.
4) Pacific Northwest Plant Disease Handbook, 1994.
5) Wiese, M.V. 1977. "Cephalosporium stripe" Compendium of Wheat Diseases. APS Press, St. Paul, MN, pp. 26-27.
6) Burns, E.E. and Mathre, D.E. Plant Disease, "Cephalosporium stripe of wheat", Montana State Extension ServiceApril 1975, Leaflet No. 77.
7) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, p. 55.
8) Bockus, W.W., Davis, M.A., and Todd, T.C. 1994. Grain yield responses of winter wheat coinoculated with Cephalosporium graminearum and Gaeumannomyces graminis var. tritici. Plant Disease, Vol. 78 No. 1, pp. 11-14.
9) Riesselman, J. Cephalosporium stripe of winter wheat. Montana Crop Health Report, September 20, 1991, No. 12.
10) Watkins, J.E, Boupnik, B., Boosalis, M.G. Cephalosporium stripe of wheat. NebGuide, Cooperative Extension Service, University of Nebraska-Lincoln, Bulletin G85-743, March 1985.
11) Mathre, D.E., Dubbs, A.L., and Johnston, R.H. 1977. Biological control
of Cephalosporium stripe of winter wheat. Capsule Information Series, Montana
Agricultural Experiment Station, Montana State University, Bozeman, No.
13.
Scientific name: Sclerophthora macrospora
Hosts: Wheat, barley, rice, corn, sorghum, and several perennial and annual grassses (1).
Symptoms:
Diseased plants are stunted, yellowed, and have excessive tillering and thickened, leathery or warty leaves. The heads and leaves often are variously twisted (crazy top). Plants often die before jointing (1).
Lab diagnosis: Numerous, spherical, yellow-brown oospores within diseased tissues are diagnostic. These appear similar to Pythium oospores. Cells may be hypertrophied and hyperplastic (1).
Look-alike symptoms: Phenoxy (hormonal) herbicide injury (MB).
Favorable conditions: Growth in standing water infested with the fungus favors disease development. Seedlings are more susceptible than adult plants (1).
Pattern in field: Disease pattern in the field corresponds to the presence of ditches, and lowlands near resevoirs of the pathogen in other hosts (1).
Infestation levels:
Life cycle: The causal fungus is an obligate parasite. Plants are infected by zoospores whose germ tubes penetrate the plants directly (1).
History in MT and US: Generally important only in tropical Asia or in localized water-logged areas (1).
Crop losses: Downy mildew is not economically important in Montana. If present, the disease can be damaging to individual plants, but the frequency of infection is low (1).
Cultural controls: Manage water in field by draining low-lying areas. Rotate wheat with non-cereal crops. Avoid use of soils and seed infested with host debris (1).
Resistant varieties:
Biological controls:
Chemical controls:
References: 1) Wiese, M.V. 1977. "Downy mildew" Compendium
of Wheat Diseases. APS Press, St. Paul, MN, pp. 34-35.
Scientific name: Urocystis agropyri
Hosts: Winter wheat (Many grasses are susceptible to other strains of the pathogen) (1).
Symptoms:
leaf
symptoms on wheat - Washington state, 1975
Near heading, long, gray-black streaks develop on leaf blades and sheaths. The streaks are linear subepidermal smut sori which develop between leaf veins. Within a few days, the sori erupt and liberate gray-black spore masses. The fraying and splitting of the leaves often weakens them. Leaves may be twisted laterally and the plant may not head. Plants are stunted and tiller excessively (1).
Look-alike symptoms:
Favorable conditions: Low soil moisture and temperatures between 10 and 20 C (1).
Pattern in field:
Infestation levels:
Life cycle: Smut balls liberated from tattered leaves contaminate soil and seed. The spores can remain viable for 4 years on seed. Teliospores in the soil or introduced on seed produce sporidia that infect wheat coleoptiles before they emerge. The fungus overwinters as mycelia within seedlings and then it systemically invades and sporulates within the upper part of host plants in the spring (1).
History in MT and US:
Crop losses:
Controls: Controls generally are the same as those for common bunt (1).
Cultural controls: A 1- to 2-year rotation away from wheat is beneficial, but some soil-borne teliospores can remain viable for 3 yr. Crop rotations are ineffective if the contaminated seed is the source of inoculum (2). Shallow planting reduces infections relative to deep planting (1).
Resistant varieties: Resistant varieties provide the best control. Some become infected but impede the maturation and sporulation of the pathogen (1).
Biological controls:
Chemical controls: Seed treatment with systemic fungicides such as carboxin and benomyl has been effective (1).
References: 1) Wiese, M.V. 1977. "Flag smut" Compendium of
Wheat Diseases. APS Press, St. Paul, MN, pp. 23-24. 2) Lamey H.A and
McMullen, M.P. 1993. "Crop rotations for managing plant diseases" North
Dakota Extension Service. Bulletin PP-705.
Scientific name: Puccinia recondita f.sp. tritici
Hosts: Primarily spring wheat in Montana. The fungus also is weakly parasitic on barley and certain wild grasses.
Symptoms of leaf rust:
leaf
symptoms, #1
leaf
symptoms, #2
1) Circular to oblong orange-red colored pustules primarily on leaves
2) Color of pustules changes to black later in the season
3) Pustules break through epidermis of wheat and barley leaves
4) Pustules do not break through on oat leaves
Look-alike symptoms:
- Aphids (Greenbug damage) (6)
Favorable conditions:
- The most critical factors are the presence of rust spores and a favorable temperature.
- The pathogen requires temperatures between 60 and 80 F and free water on the leaf surface from heavy dew or light rain for 6 to 8 hours (1).
- In Montana, leaf rust often is most severe in spring wheat-growing regions where wheat matures late.
- Fields managed under high yield systems have a higher potential for disease pressure and yield loss.
- Early fall planting can increase infection levels of leaf rust (1).
- Relatively cool nights combined with warm days are excellent conditions for disease development (6).
- Leaf rust can be severe in fields regardless of stand density or fertility level (unlike powdery mildew)(1).)
- Infection of the powdery mildew fungus has been shown to increase the susceptibility of barley plants to leaf rust (9).
Pattern in field: Injury will be uniform through the field since the spores are disseminated easily by wind (6).
Infestation levels: Monitor field just before flag leaf emergence and again during heading to determine if leaf rust and other diseases are present and warrant control (5). Winter wheat can be infected in the fall, but usually this doesn't result in yield loss (6).
Rust pustules can cause large holes in the waxy leaf surface, thereby disrupting the ability of the plant to regulate its water supply in leaves by stomatal control. As a result, the plant cannot control the rate that it loses water (transpires) (7).
Life cycle: In Montana, the source of the rust from year to year is spores blown up from major winter wheat states in the southern plains. Leaf rust spores can be blown several hundred miles in the wind and infect most of the U.S. wheat-growing area (6).
Leaf rust 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). Leaf rust has an intermediate temperature requirement in the range of 60-72 F (7).
The pathogen overwinters on volunteer wheat, barley, and certain wild grasses in states south of Montana. Pycnidia and aecia are rare and occur mostly in Europe on the alternate hosts of Thalictrum (meadow rue) (4). In Oregon, the rust overwinters mostly in the red spore stage (2).
As opposed to other leaf spotting diseases which develop upward on the plant, leaf rust spores are dispersed by wind and generally infect the upper leaves (6).
History in MT and US: Leaf rust may be the most widely distributed of wheat diseases. Also, it is the most common of wheat rust diseases (6). The disease is most severe where wheat matures late such as in spring wheat-growing regions (4). Leaf rust is the dominant rust in the southeast where wheat is grown between October and April or May, and it has become more important on winter wheat in the southern Great Plains and the Pacific Northwest (7).
Crop losses:
- Yield loss occurs when leaf rust develops on the upper leaves, especially the flag leaf. When as little as 3% of the flag leaf becomes infected, losses can be substantial (1).
- Under North Dakota conditions, yield losses can range from 0% (resistant varieties) to 30% (severely infected, susceptible varieties)(10).
- Leaf rust can cause reduced seed test weight and reduced yield (6).
- Winter wheat can be infected in the fall, but usually this does not result in yield loss (6).
- Leaf rust can be synergistically damaging in combination with Septoria (4).
Cultural controls: Because of long-distance transport of spores, crop rotation and conservation tillage have no effect on rust fungi (7, 8).
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 (7).
SPRING WHEAT VARIETIES:
RESISTANT: Amidon, Cutless, Fortuna, Len, Lew, Stoa, WestBred Express, WestBred 926. Resistant varieties may develop yellowish-white flecks.
MODERATELY RESISTANT: Pondera. Moderately resistant varieties develop small reddish-orange pustules surrounded by a yellowish-white halo.
SUSCEPTIBLE: Glenman, Hi-Line, Thatcher, WestBred Express, WestBred 926 (3). Susceptible varieties produce abundant, relatively large reddish-orange pustules that are not surrounded by a halo (6).
Chemical controls: Foliar fungicides are available to protect those crops that still have a high yield potential (after high winds, heat, frost, etc.) and if rust continues to develop under conditions of rainfall and/or high humidities (5).
Apply fungicides for leaf rust control in Montana ONLY under the following conditions:
- susceptible variety
- high yield potential
- rust pustules are present on lower leaves
- weather conditions favor further rust development (warm days + cool nights with dew)
- and the price of wheat will pay for the cost of fungicide application (10).
Timing of application: Appropriate timing of fungicide application is extremely important. Once plants are fully headed, fungicide application wil do little good. It is probably to late to spray, if 5% leaf rust is already present on flag leaves. Since 7-10 days are necessary for leaf rust to develop, many spores will have already penetrated the leaf, producing infections that are not visible yet. However, the systemic fungicides listed below will provide some curative activity (10).
The following fungicides provide good control:
1) Triadimefon (Bayleton, 3-4 oz AI, systemic fungicide by Mobay). Bayleton may be applied once or twice, with the first application at early boot stage. A single application of triadimefon gives good control of leaf rust (1, 10).
2) Propaconazole (Tilt, 4 fl. oz., systemic fungicide by Ciba-Geigy). Tilt must be applied only once at or before flag leaf emergence. Unfortunately, this application timing restriction limits the use of Tilt because leaf rust and Septoria glume blotch usually don't become problems until after head emergence (1).
3) Mancozeb products (Dithane, Manzate, Manex II, and Penncozeb, protectant fungicides) also are labelled for wheat leaf rust control. Mancozeb fungicides generally are applied twice, the first appication as soon as the flag leaf has fully emerged (early boot), followed by a second application 7 - 10 days later. Spreader-stickers are recommended for use with mancozebs to increase coverage and longevity (5, 10)
Fungicides usually are applied by air. Good coverage is essential for control. Use 5 gallons of water per acre. If ground application equipment is used, 10 to 20 gallons of water per acre are necessary (10).
References: 1) Lipps, P.E. 1988. Wheat disease control in
Ohio. Dept. of Plant Path. Ohio State Univ. 2) Pacific Northwest
Plant Disease Control Handbook, 1994. 3) Spring wheat varieties. 1994.
Montana State Extension Service, Bulletin 1093. 4) Wiese, M.V. 1977. "Leaf
rust" Compendium of Wheat Diseases. APS Press, St. Paul, MN, pp.
38-39. 5) NDSU Pest Report "Wheat leaf rust has arrived", May 29,
1992, No. 5. 6) Guide to Herbicide Injury Symptoms in Small Grains.
Second edition. 1992. Agri-Growth Research, Inc., Hollandale, MN, pp.
79-80. 7) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management.
APS Press, St. Paul, MN, pp. 51-53. 8) Lamey H.A and McMullen, M.P. 1993.
"Crop rotations for managing plant diseases" North Dakota Extension Service.
Bulletin PP-705. 9) Bockus, W.W., Davis, M.A., and Todd, T.C. 1994. Grain
yield responses of winter wheat coinoculated with Cephalosporium
graminearum and Gaeumannomyces graminis var. tritici.
Plant Disease, Vol. 78 No. 1, pp. 11-14. 10) Statler, G.D. and McMullen,
M. Wheat leaf rust. NDSU Extension Service, Bulletin PP-589 (Revised),
August, 1989.
Scientific name: Leptosphaeria herpotrichoides
Hosts:
Symptoms
Look-alike symptoms:
Favorable conditions:
Distribution of infestation:
Infestation levels:
Life cycle:
History in MT and US:
Crop losses:
Controls:
Cultural controls:
Resistant varieties:
Biological controls:
Chemical controls:
References:
Scientific name: Microdocium nivale (formerly Fusarium nivale).
Hosts: Winter wheat, turf, many grasses
Symptoms of pink snow mold: As snow melts, a felty , white fungus mat forms over the plants. Salmon-orange masses of spores appear on dead spots on leaves, especially close to crown region. Leaves may have large, spreading tan lesions with dark borders.
Look-alike symptoms:
- Speckled snow mold ,which causes leaves to crumble as opposed to pink snow mold, which causes chlorotic and dry necrotic leaves - Sclerotinia snow mold
Favorable conditions:
- Snow falling on unfrozen ground and remaining there for 3 or 4 months favors the disease.
- Signs of the disease disappear as the temperature rises and sunlight increases (3?).
Pattern if field: Pink snow mold results in a patchy distribution of affected plants in the spring. The poor stand emergence is especially noticable in areas of the field that were under continuous snow cover (5).
Infestation levels: Entire stands of winter wheat can be destroyed by spring or stands may be thinned so severely as to require reseeding with spring wheat or another crop (5).
Life cycle: Snow molds have the unusual ability to grow at temperatures very near, at, or even slightly below freezing. Infection occurs in fall by hyphae from perithecia and by ascospores or mycelia from host debris. Primary infection occurs on leaf sheaths and blades near soil level. During cool, wet weather and under snow cover, infections expand via mycelial growth. Pink snow mold is less cool-temperature dependent than other snow molds and can attack susceptible plants during cool, wet periods in fall or spring (4).
History in MT and US: More common at relatively high altitudes in certain intermountain valleys in Montana. Snow molds are especially common in the northwestern and northeastern states, British Columbia, southwestern Alberta, and Ontario. Snow molds are not common in the Great Plains where snow either blows away or the ground remains frozen beneath the snow (5).
Crop losses:
Cultural controls: In Idaho, rotation to legumes has reduced the incidence in the next wheat crop (3).
Resistant varieties: Spraque and Luke (soft winter wheats) (3). All hard red winter wheats are susceptible.
Biological controls:
Chemical controls: Charcoal as coal dust applied on snow cover to speed snown melt is helpful (3).
References: 1)? 2) Plant Disease Alert, KSU, 3/12/93.
3) Pacific Northwest Plant Disease Control Handbook, 1994. 4) Wiese,
M.V. 1977. "Pink snow mold" Compendium of Wheat Diseases. APS Press,
St. Paul, MN, pp. 32-33. 5) Cook, R.J. Veseth, R.J. 1991. Wheat Health
Management. APS Press, St. Paul, MN, p. 45.
Pictures of powdery mildew:
intermediate
resistant reaction on leaf
condia
of Erysiphe graminis f.sp. tritici
Scientific name: Erysiphe graminis f.sp. tritici
Hosts: Wheat (each pathovar is very host specific). The powdery mildew that infects barley will not infect wheat and vice-versa.
Symptoms:
1) Light-green flecks on the leaves and stems and sometimes the heads.
2) The flecks gradually develop into white-gray cotton-like patches. Small, dark fruiting bodies (cleistothecia) develop inside the lesions.
3) Yellow, browning and drying out of leaf tissue soon follows (11).
Look-alike symptoms: Herbicide injury due to bipyridyliums (Avenge, Cyclone, Gramoxone, Gramoxone Extra, Reglone, Sweep) (11).
Favorable conditions:
- The powdery mildew fungus requires temperatures between 60 and 77 F and relative humidity aboove 90% (3). (infection occurs at 40-70 F, optimum temperature is 68 F, infection stops at 80 F (11)).
- Conditions rarely extend into June since symptoms decrease with warmer, drier weather.
- In Montana, mildew can be a problem in fields that are flood irrigated or where heavy dews occur (1).
- Excessive nitrogen favors the development of powdery mildew by encouraging a dense canopy of succulent leaf tissue. The increased humidity favors mildew development and the N-rich tissue is more susceptible to infection (3).
- Generally, powdery mildew is more severe in no-till fields due to survival of mildew on plant residue (3).
- Conditions are even more favorable if wheat is under continuous production under no-tillage (3).
- Infection by the powdery mildew fungus has been shown to increase the susceptibility of barley to leaf rust (13).
Pattern in field: When present, powdery mildew usually occurs over the entire field.
Infestation levels: The disease is most severe when seedlings are infected and conditions favorable for disease development continue through flowering (5). However, it is more common in Montana for powdery mildew to be present on the lower leaves only. Warmer temperatures usually prevents development on the upper leaves (11).
Life cycle: The powdery mildew fungus overwinters on wheat stubble and straw or on volunteer wheat (5). It also can overwinter on wheat stubble as spores contained within protective structures known as cleistothecia. The epidemics of powdery mildew in the eastern states are thought to start from infections by windborne spores released from overwintered cleistothecia (12). Wind-borne ascospores from residue are the primary inoculum (5). Unlike spores from the rust fungi, spores from powdery mildew cannot blow for long distances. Usually, they blow only from field to field within a certain area (11). The powdery mildew fungus is entirely superficial except for the haustoria which penetrate the epidermal cells (5). The fungus "steals" nutrients, reduces photosynthesis, and increases respiration and transpiration in the host plant.
History in MT and US:
Crop losses: Powdery mildew commonly occurs on lower leaves in wet conditions, but seldon reduces yield unless it moves to flag leaves or heads wherein yield losses can exceed 40% (1). Yield losses occur in relation to the intensity of attack and are measured as reduced head numbers and kernel weights (5).
Cultural controls:
1) Don't overfertilize with nitrogen since this encourages heavy, succulent susceptible growth.
2) Lighter seeding rates improve air circulation and decrease disease.
2) Crop rotations and clean plowing reduce the amount of overwintering inoculum in the field although air-borne inoculum can negate these practices (5, 11).
Resistant varieties: Resistant varieties provide a good defense against powdery mildew. As with the rust fungi, multiple races of powdery mildew exist. Therefore, in breeding for resistance, it is necessary to use multiple sources of genetic resistance effective against all races of the pathogen common in the area (12). The host cuticle can serve as a mechanical barrier or invaded cells may not support the fungus which quickly dies after penetration. Some varieties permit colonization, but not sporulation. The introduction of new races can break down these lines of defense (5).
Biological controls:
Chemical controls: Chemical control of powdery mildew under Montana growing conditions is very rarely warranted (14). Foliar 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 and severe 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) There is enough soil moisture to achieve the yield potential (6).
If these conditions are present, the following summarizes foliar fungicide options:
1) Tilt (propaconizole, 41.8%; by Ciba-Geigy)
2) Bayleton 50 DF (triadimefon, 50%; by Mobay)
3) Benlate 50 DF (benomyl, 50%; from xxxxx)
GROWTH STAGE
PRODUCTa RATE/ACRE OF APPLICATION INTERVALS
Tiltb 4 oz. early flag leaf 1 applic. ony emergence allowed
Bayletonc 2-6 oz. boot stage or up 1-2 applic. to 21 days before harvest
Benlated 0.5-1.0 lb. Boot and 14 days 1-2 applic. after
cTilt also controls tan spot, septoria leaf blotch, leaf rust, and stem rust.
dBayleton also controls tan spot, septoria leaf blotch, leaf rust,
and stem rust and barley scald. However, Bayleton is no longer labelled
for use on barley although existing stocks may be used up (9,10).
References: 1) Riesselman, J. Powdery mildew in wheat. Montana Crop Health Report, May 18, 1990, No. 3. 3) Lipps, P.E. 1988. Wheat disease control in Ohio. Dept. of Plant Path. Ohio State Univ. 4) Pacific Northwest Plant Disease Control Handbook, 1994. 5) Wiese, M.V. 1977. "Pink snow mold" Compendium of Wheat Diseases. APS Press, St. Paul, MN, pp. 32-33. 6) 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 Research, Inc., Hollandale, MN, pp. 81-82. 12) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, p. 53-54. 13) Bockus, W.W., Davis, M.A., and Todd, T.C. 1994. Grain yield responses of winter wheat coinoculated with Cephalosporium graminearum and Gaeumannomyces graminis var. tritici. Plant Disease, Vol. 78 No. 1, pp. 11-14. 14) Riesselman, J. Cereal disease update. Montana Crop Health Report, June 15, 1987.
