The most common disease of cucurbits is powdery mildew.
The most common disease of cucurbits is powdery mildew. It attacks all species but there are many resistant varieties of melon and cucumber, and in time more species will incorporate resistant genes. This fungus, unlike most, doesn’t depend on free water on the leaf surface to infect the plant. It prefers fairly warm conditions and high humidity, but can also infect plants at fairly low humidity. It affects both the underside and tops of leaves and stems. Even fruit can be attacked on occasions.
There are a couple of species of the fungus, which are difficult to distinguish from one another, and many different strains. Resistance to specific strains is often listed in melons. Many crops and weeds are susceptible, making a ready source of inoculum at just about any time. All the fungus needs is a crop at a susceptible stage and the right climatic conditions. It’s generally a bigger problem later in the season and in older plants. High humidity is more favourable for the survival of the conidia which spread the disease, but lower humidity is more favourable for colonisation, sporulation and dispersion.
Rain and free water is less favourable for the disease but it will occur with or without dew. The disease can spread rather rapidly. When infected, symptoms will appear after three to seven days. Dense foliage and low light intensity are also favourable conditions. This disease should be nipped in the bud as soon as symptoms appear. Not only will an infection reduce yields, it will predispose the crop to other diseases and markedly affect yield quality.
Melons, particularly, lose sugar content when leaves are infected. There will often be symptoms when the melons are just about reaching maturity. The fruit will still look good but there will be problems with shelf life. Fortunately a number of systemic products rather effectively control this disease on cucurbits. Non-systemic fungicides can be used, but the problem is spraying effectively enough to wet both leaf surfaces. This is easier with greenhouse cucumbers than with field crops.
As the fungus has a short lifecycle, it can also rapidly build up resistance if one product is used too often. Talk to your pesticide representative about slowing this process down. – Bill Kerr ((016) 366 0616 or e-mail [email protected]). |fw