Small Grains XII

Downy Mildew (Crazy Top)

Ronda Koski, Ned Tisserat, and Mary Burrows

Cause: Sclerophthora macrospora, a soil-borne, obligate parasitic, fungus-like pathogen.

Hosts: The pathogen can infect wheat, barley, rice, corn, oats, sorghum, and several perennial and annual grasses (over 140 species of Poaceae).

Occurrence: Disease is not economically important in the U.S. A., except in localized areas subject to flooding.

Key Symptoms:

Wheat plants exhibit abnormal growth with distortion of upper leaves and spikes (thus the common name, “crazy top”).
Stunting, yellowing, and excessive tillering of infected plants.
Thickened, leathery, or warty leaves.
Infected plants may develop variously twisted heads with a proliferation of flower parts, not form heads or seeds or die before jointing.
Distorted growth resembles phenoxy herbicide injury.
Symptomatic plants often occur along the edge of a wet area or area of standing water.

Significance:

Losses from downy mildew are seldom severe enough to warrant control in wheat.
Although the causal fungus is incapable of reproducing in the absence of a host plant, the pathogen can persist in dead host tissue (infested crop residues) in the soil as thick-walled oospores that remain viable for several years.
Decay of infected plant tissues releases thick-walled oospores into the soil, where they can persist for several months (even in well-drained soils).
Oospores are carried from one field to another in diseased plant residues and soil, in seed grain, and by the wind, surface runoff water, and tillage equipment.
Oospores germinate in water or wet soil, giving rise to sporangiophores that produce characteristic lemon-shaped sporangia.
Sporangia are formed and infection occurs over a wide range of soil temperatures (44.0 – 88.0 F or 6.0 - 31.0 C).
Motile zoospores serve as the primary inoculum, and are disseminated in moving water.
Following infection, the fungus develops systemically within the plant, becoming most abundant in actively growing tissue.

Favorable Conditions:

Flooded areas of fields near ditches and in lowlands where the pathogen may occur on other hosts.
Cool to warm temperatures 10-25 degrees C (50-77 degrees F) and free moisture.
Seedlings are more susceptible than mature plants.

Management Approaches:

Improve soil drainage to better manage water movement.
Rotation to non-host crops may reduce soil- and residue-borne inoculum.
Good sanitation of equipment when moving between fields.
Use good quality seed. Fungicide seed treatments are not effective.
Seed treatment is not effective in controlling downy mildew

Laboratory Identification: With aid of a dissecting microscope, patches of milky-white, asexual zoosporangia may be seen on infected leaves. With aid of a compound microscope:

Numerous oospores develop within infected, senescing leaf, glume, and culm tissues.
Oospores are pale-yellow spherical (40-70 um in diameter), and have smooth walls.
Substomatal sporangiophoric pads may be seen in senescing diseased leaves and glumes.
To observe oospores and substomatal sporangiophoric pads, first decolorize leaf tissues and then stain these tissues with acid fuchsin.
Determinate sporangiophores develop from germinated oospores in water or wet soil.
Lemon-shaped sporangia (zoosporangia) (43.0 - 64.0 X 60.0- 100.0 um) develop on determinate sporangiophores.
Within 1-2 hours of formation, each sporangium liberates abundant (30-90) motile zoospores (9.0-12.0 um), each with 2 flagella.
Sometimes zoospore formation is aborted and secondary sporangia may be produced.
Sporangia development is optimal at 10-25 degrees C (50-77 degrees F), with lower limit near 7 degrees C (44.6 degrees F) and upper limit near 31 degrees C (87.8 degrees F).
Under optimal conditions, within 2 hours of release from the sporangium each zoospore germinates to form a slender germ tube (2.0-3.0 um wide) and non-septate (coenocytic), infectious hyphae (2.0-50.0 um wide) that penetrate wheat tissues directly, giving rise to infectious hyphae that proliferate intracellularly and intercellularly.

Agrichemicals:

Product List for PEST:


Pesticide	Product/Acre	Preharvest interval, remarks
Potassium Bicarbonate	See labels
Mono- and dipotassium salts of Phosphorous Acid	See labels

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.

Categories: Small grains, Diseases, Downy Mildew

Date: 09/18/07

Supported in part by:
Western Region IPM Center, EPA Region Vlll, National Plant Diagnostic Network, Great Plains Diagnostic Network, USDA CSREES, Colorado State University, Montana State University, South Dakota State University, the University of Nebraska - Lincoln,and the University of Wyoming.