Downy mildew, caused by the pathogen Peronospora viciae, is a common disease of peas in Victoria, South Australia and Tasmania. Downy mildew is one of the most common fungal diseases of peas and is favoured by wet, cool seasons. Night temperatures below 10°C and morning dew promotes the disease. The disease also impairs wax formation on the leaves and makes plants very susceptible to herbicide damage. Systemic infection can lead to the appearance of the disease late in the season if conditions are conducive, but yield losses due to downy mildew arise from the stunting of plants early in their growth, or from complete loss of seedlings. Substantial losses are likely to occur in cooler districts.
The disease is most common soon after emergence, but may affect plants at any growth stage during periods of moist, cool weather. Early infection causes systemic infection in plants that are a sickly yellowish-green and severely stunted and distorted. The undersides of the leaflets, in particular, are covered with a fluffy mouse-grey spore mass, (Figure 7.8). Infected plants may turn chlorotic while producing an abundant source of spores for secondary infections.
Secondary infection is localised in upper leaves, stems, tendrils and pods and results in the appearance of isolated greenish-yellow to brown blotches on the upper leaf surface, (Figure 7.9). On the lower surface directly below the lesions are masses of mouse grey fruiting bodies that produce spores under wet and cold conditions. Infected pods are deformed and are covered with yellow to brownish areas and superficial blistering. The fungus usually affects the lowest leaves and pods, (Figure 7.10).
Figure 7.8 Thick grey fungal growth on lower leaf surface, this is typical of downy mildew
Figure 7.9 Upper leaf surface turns yellow above fungal growth
Figure 7.10 Downy mildew on older plants
The fungus that causes downy mildew survives in the soil and on pea trash and can be seed-borne.
Infected seed can act as a primary source for systemic and local infections. The disease can develop quickly when conditions are cold (5 - 15°C) and humid over 90 per cent for 4 - 5 days, often when seedlings are in the early vegetative stage. Individual seedlings become infected and act as foci of infection from which the disease spreads. Rain is the major means of spore dispersal and infection. Heavy dew will promote sporulation. Dry, warm weather is unfavourable for the disease. Systemic infection of plants can lead to the disease developing late in the season if conditions are favourable, (Figure 7.11).
Most of the damage from this disease is from stunting plants early in their growth or death of 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.
Figure 7.11 Disease cycle of downy mildew on field peas. Illustration by Kylie Fowler
Growing a resistant variety is the most effective means of controlling downy mildew in districts prone to this disease. There are two strains of the downy mildew fungus. The parafield strain that is considered as a non-virulent strain infects all conventional type tall field peas such as parafield and alma. Whereas the new kaspa strain is more virulent and can infect both conventional type, older field pea varieties as well as newer semi-leafless varieties such as kaspa and PBA Oura.
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 7.1.
Seed dressing with metalaxyl or oxadixyl (Group 4 systemic phenylamide fungicides) can be effective. 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 are recommended for districts where downy mildew occurs in most years. Not all fungicide seed dressings have activity against downy mildew. For more information on seed treatments, see Pulse Australia’s Pulse Seed Treatments and Foliar Fungicides.
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.
BACTERIAL BLIGHT OF FIELD PEAS
POWDERY MILDEW OF FIELD PEAS