Don’t get caught out with downy mildew

Downy mildew is less of A problem in cabbage than it used to be. With resistant varieties available, susceptible varieties don’t have to be planted in vulnerable areas any more.

- Advertisement -

Still, there are sometimes outbreaks when the weather changes, as this disease prefers cool and moist conditions.

Symptoms of downy mildew
The disease first manifests on older leaves. with yellowish lesions on the leaves’ upper sides, and fewer lesions on the leaves higher up the plant. Underneath the leaves will be greyish, fluffy mildew under these patches.

In the few cases where spraying is necessary, be sure to use a systemic product. It’s almost impossible to cover the lower leaves adequately with a spray apparatus, so contact fungicides can be ruled out. Trans-laminar systemic products are the norm. Downy mildew is a bigger problem in seedlings, where it’s prevalent and destructive. Young seedlings suffer a huge setback, as they’re made more vulnerable to damping off, uneven growth and much-delayed maturity.

- Advertisement -

If seedlings are infected at the transplanting stage, the disease will generally stop after transplanting due to the change in microclimate. With seedlings, when conditions are likely to favour the disease, it’s advisable to use preventative sprays with a contact product to prevent the fungus becoming established.

Preventative action
Most seedling growers have spray equipment that lets them wet the underside of the leaves. Mist blowers at the right application angle can be very effective, but ensure the undersides are wet just after spraying. For the product to be effective, the plants must look saturated. Electrostatic sprayers electrically charge the droplets to make them statically attracted to the plants, so more of the fine droplets reach the underside of leaves.

When using a preventative application, alternate products so that different groups of fungicides are used. This prevents the fungus building up resistance. Speak to your crop protection representative for the most up-to-date information. Only consider using a systemic product on seedlings during an outbreak.

These products are best used as a last resort to maintain their efficacy when really needed. But overuse eventually put an end to its effectiveness. When the first systemic product for this disease, Metalaxyl, came out, it was so effective that even spray drift onto infected plants cleaned them up immediately. Overuse curtailed the effective life of this and other products.

Downy mildew on cauliflower
Downy mildew is more of a problem on cauliflower than on cabbage in open land conditions. Be on the look out for symptoms on the this crop’s leaves as, if it spreads into the developing curd, it will render the crop unmarketable. It can even spread down into the floret stems and form dark lesions that cause the curd to quickly rot. The canopy of the cover leaves is meant to protect the curd from sunlight, but also forms a perfect micro climate for the fungus.

Again, remember that some varieties are more resistant to the disease when planted during the vulnerable season. As with most diseases, be vigilant and aware of climatic conditions which favour the disease. Do frequent inspections so you can respond to potential threats before it is too late. Successful farmers have a sense of timing and anticipation, and don’t procrastinate.