Powdery mildew is a common disease of cucurbits under field and greenhouse conditions in most areas of the world. Podosphaera xanthii and Erysiphe cichoracearum are the two most commonly fungi causing cucurbit powdery mildew.
Quantity of yield is reduced due to a decrease in the size or number of fruits or a decrease in the length of the harvest period. Premature senescence of infected leaves can result in reduced market quality because fruit become sunburnt or ripen prematurely or incompletely. Such fruit have poor storability, low soluble solids with consequent poor flavor, poor rind color and shriveled, discolored handles. Stress from disease can lead to imperfections on fruit rind such as speckling, raised indentations, and oedema. Powdery mildew infection predisposes plants to other diseases, e.g., gummy stem blight.
DISEASE CYCLE
The fungus is an obligate parasite and cannot survive in the absence of living host. The primary initial inoculum is believed to be airborne conidia dispersed potentially long distances from other affected crops.
Powdery mildew develops quickly under favorable conditions because the length of time between infection and symptom appearance is usually only 3 to 7 days and a large number of conidia can be produced in a short time.
Conditions favoring infection include a dense plant canopy, low intensity light, high nitrogen fertilization, and high relative humidity (although infection can occur at relative humidity of less than 50%). Optimum temperatures for disease development are 68-80° F, and temperatures of 100° F or above stop Powdery mildew development.
SYMPTOMS & SIGNS
The symptoms of a powdery mildew are often easier to identify than symptoms of other diseases because powdery mildew forms obvious patches of whitish mycelium (resembling talc) on upper and lower leaf surfaces, petioles, and stems.
First noted on the older leaves, powdery mildew appears as pale yellow spots on stems, petioles, and leaves. These spots enlarge as the white, fluffy mycelium grows over plant surfaces and produces spores, which give the lesions a powdery appearance.
Affected leaves become dull, chlorotic, and may show some degree of wilting and eventually they become brown and papery. Yellow spots may form on upper leaf surfaces opposite powdery mildew colonies. The older plants are affected first.
DISEASE CONTROL
A programmed scouting for symptoms in cucurbit plants is always necessary to detect the beginning of a powdery mildew infection so that a pertinent treatment can be initiated. Strategies for the control of the disease involve an integrated approach using a combination of several practices.
Healthy and vigorous plants grown under a good nutritional program and suitable sanitary conditions are less susceptible to powdery mildew infection than plants under nutritional stress.
Chemical control method
Powdery mildew develops more on the lower leaf surfaces, thus a successful fungicide program requires controlling the pathogen on both leaf surfaces. An important component of fungicide programs are materials which can move to the lower surface (systemic or translaminar). Systemic fungicides, due to their single site mode of action, are prone to resistance development and the powdery mildew fungi have the ability to develop resistance to these fungicides. Resistance management can be achieved by alternation of several fungicides within a crop’s season.
The following fungicides are recommended for use in prevention and eradication of powdery mildew in cucurbits;
- ABSOLUTE STAR 400 SC 8ml in 20L
- EXPLORER® 3 SL 20ml in 20L
- MILESTONE 250 SC 10ml in 20L
- TRUSTMATE EXTREME 300 SC 5ml in 20L
- RANSOM® 600WP 15g in 20L
- MILESTONE EXTREME 390 SC 4.5ml in 20L
- CHARIOT® 500SC 20ml in 20L
Non-chemical control methods
- Proper crop spacing to ensure free air circulation
- Choose planting sites with good air movement and free of shade.
- Separate successive cucurbit plantings physically to prevent older plants from serving as an inoculum source for main crop.
- Adequate field and green house ventilation
- Minimizing field movements from infected areas to non-infected areas
- Field sanitation/ removal of affected plants and debris
- Use resistant varieties
- Avoid overhead irrigation to keep foliage dry
Tips!
- Whenever spraying, it is advisable to mix the fungicide with INTEGRA 3ml in 20L, which improves the efficacy of the fungicide by acting as a sticker, spreader and penetrant.
- Timely control of the disease helps reduce/prevent losses attributed to its infection.
- Fungicides should be reapplied after 1-2weeks.
- A proper nutrition builds/boosts the plants’ immunity against infections.
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