Plant growth, flowering and fruiting can be affected by mildew infections but some simple steps help to overcome the problem, writes PETER CUNDALL
Plant proteas to add colour in winter.
Fortunately when mildew strikes late in the season, little damage will occur
MILDEW problems on plants seem to get worse in autumn, although they can strike at any time during the growing season. Mostly they appear as insignificant dots or blemishes on young or old foliage then spread rapidly.
Fortunately when mildew strikes late in the season, little damage will occur. The worst mildew problems occur in spring or early summer. This is when vigour, flowering and fruiting are seriously retarded by this disease.
Basically, there are two main mildew groups. Powdery mildew and downy mildew look different from each other so are easily identified.
Powdery mildew looks like white or pale grey ash covering young leaves, green stems and occasionally fruit, and can strike any time during active plant growth.
This mildew usually survives winter. During late winter the youngest shoots of vulnerable apple varieties such as Jonathan often show signs of where the disease has over-wintered in infected shoots, which have a thin, shrunken appearance.
In spring the first leaves are immediately covered with powdery mildew and shrivel within days.
If not treated, the disease persists throughout summer and autumn, weakening trees and massively reducing apple yields. The treatment is to cut off all infected shoots, collecting all infected material in a wheelbarrow so it can be carted away.
Other forms of powdery mildew occur on late-sown peas as leaves and pods become white with the powder. The disease usually strikes pea plants which are overcrowded, as temperatures rise in summer. Luckily the peas within infected pods are perfectly disease free and edible so there is little point in spraying. Better to sow pea seeds in August, so harvesting can begin in December.
Ornamental plants which become easily infected with powdery mildew include semi-evergreen rose varieties such as the common Banksia rose. It is the leaves on tangled growth which come under powdery mildew attack, even worse if roses are in too much shade.
Pruning and thinning congested growth gives excellent control. It is best carried out in early January but any time of the year gets good results. Drastically thinning, rearranging and spreading branches to allow air to move freely through plants makes conditions difficult for powdery mildew.
Right now in many autumn vegetable gardens, powdery mildew has already started to show up on parsnip, cucumber, pumpkin and zucchini leaves. Parsnips sown in mid-spring will have roots big enough to eat. So do nothing. Just leave parsnips in the ground until they have been sweetened by frosts, then enjoy them through winter and spring.
Younger, smaller parsnips can be sprayed with a solution of one part of milk to 10 parts of water to provide good control.
Many pumpkin and especially zucchini leaves are already showing signs of mildew infection. This is no big deal. It usually happens as the plants age and mature, just as we get greying hair as we get older. With zucchini plants, the first leaves to be attacked are older, lower leaves. Cut them off at the first signs of mildew infection and this prolongs the life and fruiting of the plants.
Downy mildew is a different type of fungal disease. It attacks leaves of grapes, cucumbers, brassicas, lettuce, onion, pansies and even beetroot. Dark, slightly sunken patches appear on the upper surfaces of leaves. On the undersides, clusters of glistening purplish or silvery down appear.
Spraying grapevines with Bordeaux or Burgundy mixtures as a preventative cover has been fairly successful over many years.
Opening up plants to lots of free air circulation also helps stop mildew from taking hold. A simple, safe means of control is to dissolve two teaspoons of bicarbonate of soda in a litre of water, add a drop of household detergent and spray at the first signs of infection.