Downy Mildew of Brassicas

Peronospora parasitica

Description:

A fungal disease that cause cream to white fluffy growth on the underside of the leaves and yellow speckled patches on top that become sunken, angular, brown to black patches with tiny black specks on mature leaves. There may also be a black speckling and puckering of the topside of cotyledons. Seedlings may be killed.
If conditions become dry fungal growth stops and infected areas dry out to brownish irregular patches
Pale brown patches may develop on the surface of Broccoli and Cauliflower heads and there may be dark brown to black streaks on the stalks.
Cabbages and Brussels Sprouts may have purple black spots on the outside leaves.
Stocks develop yellow patches and the whole leaf may yellow and fall. Defoliation may kill the plant in severe cases. It is mainly a problem of seedlings that shows as a furry white hyphal growth on the underside of leaves followed by wilting and death.
In Canola, yellow blotches appear on the topside of leaves with white mealy growth on the underside. It is usually seen after stem elongation in thick crops, under cool wet conditions on leaves that contact the soil or other leaves.

Species Affected:

Brassicas, Canola.
Broccoli, Brussels Sprouts, Cabbages, Cauliflowers are susceptible.
A different race of the same disease attacks Stocks.

Biology:

More common in cool, humid and moist conditions caused by the weather or density of the crop.
Survives for many months on crop stubble or on Brassica weeds.
Spread by wind.
Requires moisture on the leaf for infection.
Spore release usually peaks at mid morning.

Life Cycle:

Origin and History:

Distribution:

Significance:

Very common in Canola but rarely causes economic losses. Rapidly growing crops usually out grow the disease.

Management and Control:

Avoid continuous cropping with Brassicas.
Avoid irrigation during the morning.
Sow seed more thinly, increase circulation.
Destroy diseased plants.
Eradicant fungicides are effective but the disease may become resistant to them.
Protectant fungicides should be applied at the first sign of the disease and applied regularly while cool, humid, and moist conditions that favour infection prevail.

Related and Similar Species:

References:

1292

1291

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.HerbiGuide.com.au for more information.