Downy mildew is one of the major and serious horticultural crop disease that has troubled many. Its caused by fungi in the oomycetes found in the genera peronospora. You may wonder why its known as downy mildew, right? Here i make it clear for you, turn over the affected leaves-you will observe downy tuft spores which are white-purple or grey conidia spores that can be easily dislodged and dispersed by wind or splashing water.
The disease is common during cool moist weather and tend to affect the tender above ground plant parts. Presence of water on the leaf surfaces is one of the key condition for rapid disperse of the fungal spores. It’s more prevalent under relative humidity of about 85% and above, and a temperature ranging from 10-22 degrees celsius.
Symptoms of attack
Symptoms primarily appear on the leaf surfaces as purple patches or brown colourations bounded by the leaf veins. Affected leaves may fall prematurely and severe defoliation may be witnessed in later stages if control measure are not executed at the right time.
Disease cycle and development
Disease inoculum overwinters on or in plant parts as mycelium or oospores, that is thick walled, gum resembling structures which become active when conducive propagation conditions prevail (85%>relative humidity and cool conditions). Prolonged leaf wetness, warm and humid weather conditions accelerates the rate of disease propagation. Disease propagation or sporulation (production of secondary spores) take 7-10 days but can take less days (4) during warm humid weather. Incase conducive conditions for spore propagation are not available, then the spores survive in plant debris, weeds or even in the soil awaiting for susceptible plants.
General management/Avoidance measures
- Avoid prolonged leaf wetness, practice drip irrigation method instead of overhead that leads to foliage wetting predisposing plants to downy mildew pathogen.
- Increase air circulation through plant pruning and thinning.
- In greenhouse environments, maintain humidity below 85% by opening ventilation, increase air circulation using air flow fans, good plant spacing, increase temperatures using air heaters
- Proper weather observation, if humidity approaches 85% and temperature drops to about 10-22 degrees then protect your crop using copper based fungicides eg Ridomil gold.