Ascochyta Leaf Spot
Ned
Tisserat Kansas State University, Plant Pathology
Mary Burrows, and Ronda Koski,
Cause: Ascochyta tritici (asexual state) or Didymella
exitialis (possible sexual state)
Hosts: The pathogen can infect most members of the grass family
(Poaceae) throughout the world.
Occurrence: Ascochyta tritici persists in host debris as
mycelium and pycnidia. Spores (conidia / pycnidiospores) released during wet
weather conditions are suspected to be the primary inoculum. Pathogen
distribution and disease frequency may be greater than reported because grasses
throughout the world are susceptible to Ascochyta tritici.
Key Symptoms:
- On leaves of wheat plants,
initial symptoms appear as chlorotic flecks.
- As the disease progresses,
chlorotic flecks develop into distinct chlorotic lesions that may be
ellipsoidal or round, typically 1-5 mm in width.
- Over time, lesions become
diffuse and grayish-brown in color (similar to those caused by Septoria
nodorum).
- Submerged black fungal
structures (pycnidia) develop within necrotic lesions; these appear as
tiny black dots.
Significance:
- Factors including disease
that reduce green leaf area, especially on the flag leaf, are likely to
reduce grain yields, thus yield reductions may occur in wheat plants
infected with Ascochyta tritici.
- Losses are seldom severe
enough to warrant chemical control.
- Ascochyta leaf spot is
generally of minor economic importance in Europe, Japan, New Zealand, and
North America, and is often overlooked as it can occur in association with
other leaf spot diseases.
Favorable Conditions:
- Development Ascochyta leaf
spot is favored by warm, moist weather conditions, accompanied by periods
of high humidity.
- Dense foliage and foliage
in contact with the soil also favor this.disease.
Management Approaches:
- Control measures prescribed
for Septoria diseases should be effective controls for Aschochyta leaf
spot, these control measures include:
- Use of resistant cultivars
(if available); in general, early-maturing cultivars are most susceptible
to Septoria diseases.
- Use of pathogen-free seed
- Destruction or avoidance
of infected wheat residues and volunteer wheat by crop rotation or
tillage. Stubble mulch and minimum tillage practices increase the risk of
disease.
- Cultural practices that favor
air movement through the field; these include wide row spacing and
adequate, but not excessive fertilization to limit foliage density and
relative humidity in the crop canopy.
- Proper seeding date for
your area. Late planting may encourage disease.
Laboratory
Identification:
- With aid of a dissecting
microscope ~ look for black pycnidia in necrotic tissue of lesions.
- With aid of a compound
microscope~look for black globose ostiolate pycnidia mostly submerged
in the host tissue except for a papillate projection that will rise about
the surface of the host tissue.
- When grown on various artificial media, Ascochyta tritici will produce immersed
black globose ostiolate pycnidia .
- Pycnidia measure 103 - 320
X 124 - 172 um.
- Pycnidia give rise to
straight, oblong, hyaline conidia (pycnidiospores) (3 - 6 X 14 - 27 um)
that typically have 1 median septum.
Agrichemicals:
Product List for PEST:
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Pesticide
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Mode of Action
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Product/Acre
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Preharvest interval, remarks
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Potassium Bicarbonate
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See labels
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The information herein is supplied with the understanding that no
discrimination is intended and that listing of commercial products, necessary
to this guide, implies no endorsement by the authors or the Extension
Services of Nebraska, Colorado, Wyoming or Montana. Criticism of products or
equipment not listed is neither implied nor intended. Due to constantly
changing labels, laws and regulations, the Extension Services can assume no
liability for the suggested use of chemicals contained herein. Pesticides
must be applied legally complying with all label directions and precautions
on the pesticide container and any supplemental labeling and rules of state
and federal pesticide regulatory agencies. State rules and regulations and
special pesticide use allowances may vary from state to state: contact your
State Department of Agriculture for the rules, regulations and allowances
applicable in your state and locality.
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Categories: Small grains, Diseases, Ascochyta leaf
spot
Date: 09/18/07
