THE powdery mildew is a wide-spread fungal disease of approximately 10,000 plant and tree species, belonging to more than 1,600 genera. In Pakistan, it is reported on near- about 70 different plants and trees, including cereal crops, pulses, oilseeds, vegetables, fruits, medicinal plants, fodder and timber.

The disease occurs as white talcum powder mass on fleshy green plant parts. The severity of the disease varies with the fungal species, depending on many factors, including variety, age and condition of the plant, and weather condition during the growing season. It becomes severe in warm and dry climates and results in reduction of growth and yield of crops, sometimes by as much as 20 to 40 per cent. The fungus grows and infects its hosts in the absence of free water, survive on infected plant parts and in debris such as fallen leaves and spreads by spores blown by wind. Burning infected plants and other cultural practices can help minimise the incidence, but the disease can better be controlled through frequent sprays of fungicides.

An average temperature and relative humidity favours growth, development and multiplication of the fungus. The first quarter of the current year 2007 was favourable for the growth of this fungus and was a potential threat to the crops of mango, oilseeds vegetables and cereals. However, as the disease got a very little favourable period to flourish, there was little impact of the fungus on crops.

The powdery mildew is caused by species of fungi belonging to the genera Erysiphe, Leveillula, Oidium, Podosphaera, Phyllactinia, Sphaerotheca, Uncinula and Microsphaera, members of a single family Erysiphaceae of order Erysiphales, class Ascomycetes. Some of these fungi are most destructive. In many cases one fungus attacks only one plant or crop for example Oidium mangifera infects only mango, Podosphaera leucotricha attack only apple, but do not cause powdery mildew on any other plant or tree. On the other hand, Erysiphe polygoni alone is recorded on more than 350 host species the world over.

The fungal disease appears usually as separate, round, powdery white spots. As the spots increase in size, they come together, appear as patches of cottony-white to greyish or tan, talcum powder like growth of casual fungus. Later, these turn into yellow blotches on the upper and lower surfaces of leaves, leaf sheaths and other fleshy green parts of the plant/tree. The harm includes yellowing, discoloration, stunting and distortion of leaves, buds, growing tips, pods/fruits and green parts of stem. The fungus may cause death of invaded tissue.

In case of wheat, it is most prevalent on lower leaves but can cause blighting of the upper leaves, heads and awns of susceptible cultivars, may reduces yield as high as 40 per cent, reduce grain size and may also decrease flour protein but do not affect milling and baking quality (FAO Plant Production and Protection Series No. 30, (2002).

In some cases on tomato and eggplant, yellow patches appear on leaves, but there is no powdery growth of the fungus on leaf surfaces. In most annual plants and shrubs, disease appears on the upper surface of older leaves first, which turn brown and die when heavily infected. New shoots may be infected and killed in grapes and some fruit trees. In mangoes, symptomatic powdery spots occur only on inflorescences but not on other parts, whereas young small fruits represent damage without typical white spots.

In severely infested peas, brown, pitted spots may occur on pods and the seed may be visibly affected and premature ripening may result in shrunken seed. In sunflower, the disease usually occurs too late in the season, look like very serious but is not largely damaging. In case of rapeseed and mustard, typical white powdery spots appears on all fleshy green parts of the plant, pods, leaves, leaf sheaths, pod and leaf stalks, even fleshy green portions of main stem is also covered with powdery fungal growth.

The fungus produces thread like structures (mycelium) that grows only on the surface of the plant; never enter the tissues by force themselves. The fungi feed by sending root-like structures (haustoria) into the top (epidermal) cells of the plant and develop tiny, pinhead-sized, sphere-shaped fruiting structures, initially are white, later yellow-brown and finally black, may be present singly or in a group, called over wintering or over summering bodies (cleistothecia) of the fungus.

The powdery mildew causing fungus survives on infected plant parts and in debris such as fallen leaves, but the mycelium cannot survive unless it is in living host tissue. It may produce resting structures; appear as small black dots within the white powdery patches, known as cleistothecia, which resist environmental changes. In the next season, spores (ascospores) are released from the cleistothecia and are carried by air currents to plant parts where new infections begin. During the growing season, the fungus produces asexual spores (conidia) that help the fungus to spread and build infection..

It is one of the few foliar diseases, which is prevalent in desert areas, favoured by moderate or high humidity and low light intensity conditions which are often prevalent on lower plant parts or in thick foliage and is found throughout the low desert areas most of the year.

The fungus takes 5-7 days for appearance as it can multiply rapidly when temperature is between 60 and 80 degrees F. Severity of the disease increases gradually to a point of doubling as the RH increases from 39 per cent (the lowest level that could be tested) up to an optimum near 85 per cent to 90 per cent. Therefore, coastal regions (with both temperature and humidity) are highly favourable for powdery mildew throughout much of the season.

Management Strategies: Cultivation of resistant varieties is the safest, easy and economical control of all diseases, but availability of resistant varieties is not common. Breeders must try to develop resistant varieties. Crop rotation with non-host crops is helpful in controlling the disease.

Other management strategies for the control of the disease are to collect and burn fallen leaves and prune out all infected portions as soon as the disease occurs. The removal of closely related weed hosts (spreading disease) is also important for field isolation to reduce secondary infection.

Olsen and Young (1998, Cooperative Extension, University of Arizona) recommended overhead irrigation (because consistent application of free water will inhibit spore germination or kill spores of the disease causing fungi) and to create good air flow on the canopy (because careful pruning in grapes, fruit trees and susceptible shrubs will open the canopy, increasing light and decreasing humidity, which not favour the fungi). However, temporary or minimum overhead watering should be avoided; it may help to increase infection. Avoid overcrowding of smaller plants through thinning or uprooting extra plants or by pruning woody plants for maintaining recommended spacing.

Frequent irrigation and heavy application of fertilisers may also be avoided; especially high nitrogen fertilisers encourage soft sappy growth on which disease develops.

If cultural controls fail to prevent disease or if the disease appears early in the growing season and its pressure is too vast, chemical control will be most effective if started by application of any suitable fungicide at the appearance of first powdery spots or patches on any plant, and repeated properly, during cool humid weather. The normal spray schedule should begin at the initial inflorescence appearance stage for mango and green tip bud stage for apples.

Sulphur or sulphur-based fungicides are mostly used against powdery mildew of many crops. According to Wilcox (2003, www.practicalwinery.com), “sulphur is cheap and effective, traditionally used to control powdery mildew around the world for nearly 150 years, with no development of resistance. However, because sulphur acts largely through the vapour phase, its activity is temperature-sensitive. Sulphur is relatively inactive at cool temperatures below 65 degree F, and can be phytotoxic at temperatures above 85 degrees to 90 degrees F. Resistance to some fungicides (such as Benlate, Topsin-M, Vangard) follows an “all or nothing” model. That is, nearly all individuals in the fungal population are highly susceptible to the materials when they are first introduced, but a few are virtually immune to any amount of it. These individuals build up very quickly once enough sprays are applied to kill off the susceptible population, and because the resistant population is completely unaffected by the fungicide, control failures can occur suddenly if the weather turns in favour of the disease.”

Sulphur dusts, however, are used on grape vines, apple and also be possible to treat certain vegetable crops and ornamental plants, but cautiously as sulphur is known to cause injury to some plants, as in case of chillies, which is known sulphur-shy crop. Certain cultivars of apple and vegetable crops may also be damaged by sulphur dusts. Therefore, labels should be checked cautiously about its sensitivity or experts should be consulted. Potassium bicarbonate, jojoba oil and some products containing neem oil or thiophanate-methyl as the active ingredients may also be used, but it should be discontinued if it does not seem to be controlling the disease.