Picture
#1 - general chlorosis and yellowing of whole potted plant
Pictures of Aster Yellows:
Picture
#1 - general chlorosis and yellowing of whole potted plant
Picture
#2 - 3 wheat heads, healthy (left), sterility, chlorotic, and distorted
(center and right)
Cause: Aster yellows is caused by a mycoplasmwhich is an organism that is related to bacteria, but produces symptoms more consistant with those of a virus (1).
Hosts: Aster yellows is most severe in lettuce, carrot, potato, and some ornamentals. The disease also has been described in wheat, barley, and oats (1).
Symptoms: In the early stages, yellow blotching of the leaves occurs. This is followed by a general yellowing of the leaf and a stunting of the plants, and poor head development (1).
Look-alike symptoms: Symptoms of aster yellows in wheat and barley often are confused with barley yellow dwarf and other virus diseases (1).
Lab diagnosis: Martin Draper (NDSU clinc) tested for the disease using a sectioning and staining technique (1).
Favorable conditions: Since leafhoppers are the vectors, a heavy infestation of leafhoppers in the field favors aster yellows.
Pattern in field:
Infestation levels:
Life cycle: Aster yellows is transmitted by leafhoppers. However, symptoms of the disease don't appear until 21 to 56 days after leafhopper transmission.
History in MT and US: Researchers from Canada and Minnesota described aster yellows on wheat, barley, and oats in the 1960's (1). The disease has been confirmed only rarely in the Great Plains.
Crop losses:
Controls:
References: NDSU Pest Report, "Leafhoppers and aster yellows
in small grains", June 17, 1994, Issue 7.
Pictures of Bacterial blight:
pseudomonas
on individual leaf
Scientific name: Pseudomonas syringae. See black chaff. Similar symptoms and controls.
Hosts:
Symptoms of bacterial leaf blight:
field
shot of symptoms in winter wheat
Lab diagnosis:
Look-alike symptoms:
Favorable conditions: Continuous cropping, reduced tillage, sprinkler irrigation, or long periods of wet weather.
Pattern in field:
Infestation levels:
Life cycle: Bacterial blight survives in the stubble and straw of wheat, barley, and oats (1)
History in MT and US:
Crop losses:
Cultural controls:
Resistant varieties:
Biological controls:
Chemical controls: None available (1).
References: 1) Lamey, H.A. and McMullen, M.P. 1993. "Crop rotations
for managing plant disease" NDSU Extension Service, Bulletin PP-705.
BASAL GLUME ROT (SPIKELET ROT, BASAL GLUME BLOTCH)
Picture of basal glume rot:
Scientific name: (Pseudomonas syringae pv. atrofaciens)
Hosts: Wheat, barley, some noneconomic grasses (2).
Symptoms of basal glume rot:
- Dark blotches or streaks near base of glume, streaks may extend more than halfway up glumes (1, 2).
- During wet periods, a whitish-gray bacterial exudate may form at the glume base.
- In advanced stages, kernels are shrunken and stained brown-black at their embryo end (2).
Look-alike symptoms: Black chaff (Xanthomonas translucens) (2).
Favorable conditions: Wet, mild weather during heading and grain fill.
Pattern in field:
Infestation levels:
Life cycle: The bacterium survives on host plant residues, in soil and on seed. Wind-blown dust or residue fragments carry bacteria to seeds (1).
History in MT and US: Basal glume rot occurs more commonly in nearly all wheat growing regions of the world where heading is concurrent with excessive moisture (2). The disease is not common in Montana.
Crop losses: Basal glume rot can reduce yields by reducing seed fill (2).
Cultural controls: Although moisture at the time of flowering can influence disease greatly, select clean or chemically-treated seed for planting.
Resistant varieties:
Biological controls:
Chemical controls:
References: 1) Lipps, P.E. 1988. Wheat disease control in Ohio.
Dept. of Plant Path. Ohio State Univ. 2) M.V. Wiese. 1977. Compendium
of Wheat Diseases. APS Press, St. Paul, MN, p.7-8.
BLACK CHAFF
or BACTERIAL STREAK
Scientific name:Xanthomonas campestris pv. translucens (Jones et al) Dye (8) (syn=Xanthomonas translucens pv. undulosa)
Hosts: Wheat, barley, oats, rye, and numerous grasses. Strains of X. campestris pv. translucens differ in their host range on small grains (9). For example, a strain that is strongly pathogenic on wheat may be non-pathogenic on barley and vice versa.
Symptoms of black chaff: Symptoms can occur on leaves or heads. The name bacterial streak refers to water-soaked lesions on leaves, which are the most important disease symptoms in terms of potential losses. Black chaff refers to black lesions on glumes (6).
Leaves:
- Disease symptoms rarely are seen on leaves before the heading stage (7).
- Usually, symptoms develop between the heading and flowering stages, after which they develop rapidly on all leaves (7).
- Leaf symptoms begin as water-soaked streaks. These develop into a tan to dark brown necrotic spots or streaks surrounded by lime-green chlorotic tissue bordering the lesion (1, 2).
- Usually there is no pattern on upward progression of symptoms on plants (7).
- Initial symptoms often begin on the upper leaves where the leaves bend over (i.e., middle of the blade).
Glumes:
- Leaf symptoms do not always accompany head symptoms (10)
- Brown-black streaks or blotches on upper half of glumes;
- Awns often have dark spots on them giving them an alternating green and brown pattern that gives them a "barber pole" effect (10)
- Dark "pencil" streaks may by present on the peduncle (10).
- A light purplish to yellow lesion often is present on the neck region of the culm (5).
- Do not confuse black chaff with melanism (see Look-alike symptoms
below).
Look-alike symptoms:
Symptoms are similar to those of:
- melanism (= false black chaff)
- basal glume rot (2)
- speckled leaf blotch (dry, necrotic appearance of infected leaves (8))
Favorable conditions:
- Moist, mild weather during flag leaf emergence through grain fill (1).
- Overhead sprinkler irrigation may be favorable to disease development (10).
- Black chaff is more severe after wind-driven storms since pelting of the leaves with soil particles allows entry of the bacterium (4).
Pattern in field:
Infestation levels:
- When black chaff infection occurs late in the season, damage to grain usually is minimal (10).
- Black chaff can be destructive when it occurs during early heading wherein it can cause complete sterility (6).
- Sprinkler irrigation may increase the chances of yield losses due
to black chaff (10).
Life cycle: The bacterium survives on residues, in soil and on
seed. It is spread by splashing rain, wind and plant-to-plant contact (1).
The bacteria can exist for some time in a symptomless phase on leaves of
wheat. It is when they invade wounds or natural openings in leaves or heads,
where they multiple and kill tissues, that damage results (6). Entrance
into the plant is through stomata and wounds. The bacterium lives intercellularly,
especially in creases that harbor free water (3). When wheat matures, the
bacterium is harvested with the seed. When planted, the bacterium returns
to the soil in crop residues.Seed formed in diseased heads will be contaminated
with the bacterium (6).
History in MT and US:
- Black chaff problems are sporadic in Montana.
- Severe epidemics have occurred in south-central and eastern Idaho since 1980 (2).
- Black chaff is an important disease in the southeastern US (8).
Crop losses: Damage is minimal even when symptoms are apparent.
Destructive epidemics are rare and little information is documented on
them. Often the disease is not noticed in the field (3). Entire leaves
may be prematurely killed. If this happens before the soft
dough stage, yield reductions and shriveled grain of low test weight
may result. Diseased heads mature late and may be sterile if infected before
flowering (2).Yield losses are difficult to obtain. However, some researchers
have indicated that yield reductions of up to 40% have occurred in infected
fields.
Controls: The best methd of controlling or preventing introduction
of black chaff in fields is to plant clean seed (see Cultural controls)(6).
Cultural controls: Control is difficult once symptoms are visible. The best control is prevention by planting seed produced in clean fields and to reduce the frequency of irrigation (1,2). Seed stored more than 6 months is not an important source of inoculum.Wheat or barley seed can be assayed for the presence of black chaff can submit a 1 lb sample and a $40 check payable to Bursar- University of Idaho to: Seed Pathology Lab, Dept. of Plant, Soil, and Entomological Sciences, University of Idaho, Moscow, ID 83843 (2).
Pertinent information to be sent with the sample should include:
1) Grower name and address
2) Variety
3) Source of seed and year of harvest.
4) Information of any notic of black chaff symptoms in the
field lot where this seed sample was taken (6).
Resistant varieties:
Differences in black chaff severity among varieties under natural field conditions do exist (8).
In Asia, resistant or tolerant varieties exist that can reduce or eliminate black chaff (3).
There is no evidence of the existance of races of X. campestris
pv. translucens. Therefore, cultivars rated as resistant to bacterial
streak in the field are likely to remain resistant to susceptible cultivars,
although actual disease severities may vary due to differences in inoculum
level or environmental conditions (9).
Biological controls:
Chemical controls: None available.
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) M.V. Wiese. 1977. Compendium of Wheat Diseases. APS Press, St. Paul, MN, p.8. 4) NDSU Pest Report, "Head diseases in small grains", July 24, 1992, No. 13. 5) Riesselman, JR "Black chaff of cereals", unpublished. 6) Cook, R.J. Veseth, R.J. 1991. Wheat Health Management. APS Press, St. Paul, MN, pp. 54. 7) Milus, E.A. and Mirlohi, A.F. 1993. A test tube assay for estimating populations of Xanthomonas campestris pv. translucens on individual wheat leaves. Phytopathology, v. 83 (2), pp. 134 - 139. 8) Milus, E.A. and Mirlohi, A.F. 1994. Use of disease reactions to identify resistance in wheat to bacterial streak. Plant Disease, v. 78 (2), pp. 157 - 161. 9) Milus, E.A. and Chalkley, D.B. 1994. Virulence of Xanthomonas campestris pv. translucens on selected wheat cultivars. Plant Disease, v. 78 (6), pp. 612 - 615. 10) McMullen, M. Black chaff and false black chaff of wheat. NDSU Extension Service, Bulletin PP-749 (Revised), November 1989.
