Downy Mildew of Field Peas

Note Number: AG0149
Published: June 2008
Updated: August 2012

Downy mildew is one of the most common fungal diseases of field peas in Victoria, and often causes substantial reduction in plant numbers in cool wet seasons. It can be effectively managed by using crop rotations, resistant varieties and seed dressings.

What to look for

Fig 1. Thick grey fungal growth on the lower leaf surface, is typical of downy mildew

This disease is most common soon after seedling emergence, but it may affect plants at any stage of growth especially during periods of moist, cool weather.

Plants grown from infected seed are severely stunted and distorted, and have a sickly yellowish-green appearance. The undersides of the leaflets, in particular, are covered with a fluffy mouse-grey spore mass (Figure 1). Infected plants may turn yellow while producing an abundant source of spores, which cause secondary infections.

Secondary infection results in the appearance of isolated greenish yellow to brown blotches on the upper surface of leaves (Figure 2), while on the underside of the leaf, masses of mouse-grey coloured spores are produced.

The fungus usually affects the lowest leaves and then progresses up the plant, sometimes infecting flowers and pods. Infected pods are deformed and covered with yellow to brownish areas and superficial blistering (Figure 3).

Disease Cycle

Fig 2.  Greenish yellow to brown blotches typical of secondary infection

Downy mildew, caused by the fungus Peronospora viciae, survives in the soil and on old pea trash; it can also be seed-borne. Infected seed can act as a primary source for systemic infections. Systemic infection of plants can lead to the disease developing late in the season, if conditions are favourable.

The disease can develop quickly when conditions are cold (5-15°C) and wet for 4-5 days. This often happens when seedlings are in the early vegetative stage. When individual seedlings become infected they act as a focus of infection from which the disease spreads. Heavy dews will promote release of spores, while rain is the major means of spore dispersal, and secondary infection. Dry, warm weather is unfavourable for the disease.

Economic Importance

Fig 3. An older plant effected by downy mildew

Downy mildew causes most damage by stunting plants early in their growth or by killing seedlings in more extreme instances. Generally, plants will grow away from the disease as temperatures increase in late winter/early spring without significant yield loss.

The disease also impairs formation of wax on the leaves which makes plants very susceptible to damage by herbicides. Substantial losses can occur in cooler districts.

Management

Varietal selection

Growing a resistant variety is the most effective means of controlling downy mildew in districts prone to this disease. However, there are now two strains of the downy mildew fungus. They are the Parafield strain and the Kaspa strain. The Kaspa strain is a new strain, and prior to its emergence there were commercial varieties available with resistance to downy mildew.

Now with the Kaspa strain detected in Australia, there are no commercial varieties with resistance to both strains of the fungus. The resistance of current field pea varieties to both strains of downy mildew is shown in Table 1.

Chemical control

The main effects of downy mildew can be reduced by treating seeds with fungicides. Seed treatments reduce the number of seedlings with primary infection, thereby reducing the amount of air-borne spores that cause secondary infection in the surrounding crop. Seed treatments can be beneficial, and are recommended for districts where downy mildew occurs in most years. Not all fungicide seed dressings have activity against downy mildew.

Crop rotation

Extended crop rotations and destruction of infected pea trash will minimise the risk of serious disease. Extended crop rotations allow spore numbers in the soil to decline before sowing again to field peas. A break of at least 3 years between field pea crops is recommended. Avoid sowing pea crops adjacent to last season’s stubble

Further references

More detailed information can be obtained from the DPI Information Note Series: http://dpi.vic.gov.au/

Pulse Australia
Victorian Winter Crop Summary
Winter Pulse Disorders: The Ute Guide.

Contact/Services available from DPI

DPI Field Crops Pathology, Grains Innovation Park, 110 Natimuk Rd, Horsham 3400. Tel (03) 5362 2111, or the DPI Customer Service Centre 136 186.

Acknowledgement

This Information Note was originally written by Trevor Bretag and Helen Richardson, June 2008 and was reviewed by Frank Henry, BioSciences Research - Farm Services Victoria, May 2010 and August 2012. Financial support by the GRDC is gratefully acknowledged.