HPIPM: Downy Mildew

Downy Mildew (Scientific name)

Author: Howard F. Schwartz, David H. Gent, Martha Mikkelson, and Jack Riesselman

• Source1
• Source2
• Source3

Cause

Downy mildew of pea is caused by the fungus-like organism Peronospora viciae, and occurs in most regions of the world where pea is grown.

Symptoms

Downy mildew symptoms may be local or systemic. Systemically infected plants are stunted, yellowed, distorted, and often die before flowering; abundant sporulation of the pathogen is evident on plant tissues. Local lesions are greenish yellow to brown blotches of varied shape on the upper surfaces of leaves. Opposite these blotches, fluffy, gray fruiting bodies of the pathogen are evident on the bottom side of the leaf. Leaf infections appear first on older leaves, but progress up the plant. Pods can be infected under severe disease pressure, causing yellow to brown blotches to form and deformation of pods.

Impact of Disease

The disease can be very damaging during cool wet weather, but generally causes little economic damage to pea in the High Plains.

Life Cycle of the Pathogen

The disease is often initiated by soilborne resting structures (oospores), which germinate during cool, moist weather and infect pea locally or systemically. Secondary spread of the disease occurs by asexual spores (sporangia) that are produced on infected leaves, stems, and pods. Sporangial infection can occur over a range of temperatures (34 to 68°F), but infection occurs optimally between 40 to 46°F with at least 12 hours of relative humidity greater than 90% or when free moisture is available. Sporangia survival depends on temperature, but sporangia can survive as long as 31 days at 40°F. The pathogen can be disseminated by windblown sporangia and splashing water, and survives between crops pathogenically on alternate hosts (vetch and broad bean) and as oospores in the soil. Oospores can remain viable in the soil for 10 to 15 years.

Management Approaches

Biological Control

No biological control practices have been developed for downy mildew of pea.

Cultural Management Practices

Resistant and tolerant pea varieties are available and should be planted if downy mildew becomes a recurrent problem. Long crop rotations (5 years or longer) to nonhosts can provide some disease control, but longer rotations may be necessary if severe infections occur. Deeply incorporate pea residue after harvest to reduce pathogen survival and overwintering. Avoid late season plantings and deep planting depths.

Chemical Management Program

Chemical controls are most effective when integrated with sound cultural practices. Resistance to common fungicides such as metalaxyl has been reported, and fungicides with different modes of action should be used where resistant strains of the pathogen are present.

See Pulse Crop Foliar Fungicides for fungicide recommendations.

Consult pesticide labels for additional rate, application, and safety information. Additional information and current labels can be found at http://www.greenbook.net or http://www.cdms.net.

References

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.