Diseases and Their Management

The main diseases of Cucurbitaceous vegetables and their control measures are describes as follows.

Downy mildew 

Cause: Fungus – Pseudoperenospora cubensis

Symptoms:

Downy mildew infested leaf

Leaves have a mottled appearance. Leaf spots turn a pale yellow colour, enlarge and dry out. 

**Can be confused with the bacterial disease angular leaf spot.

Favored by:

Moist conditions, such as rain, fog and heavy dews and therefore more common in districts of higher rainfall.

Control: 

·         Use tolerant varieties where available.

·         Monitor crops closely to identify the disease early in its cycle and be alert to weather conditions that may cause an early infection.

·         Use a fungicide program that allows for the rotation of protective and systemic fungicides which reduce the chance of fungicide resistance developing.

Powdery mildew

Cause: Fungus – Podosphaera xanthi

Powdery mildew infested leaf

Symptoms: White powdery spots develop on leaves. Symptoms usually develop first on the underside of leaves before covering both sides. Leaves gradually turn yellow to a papery brown.

Favored by: Warm, relatively dry conditions (unlike downy mildew). Light dews provide sufficient moisture for infection.

Control:

·         Use tolerant varieties where available.

·         Monitor crops closely to identify the disease early in its cycle.

·         Use a fungicide program involving the rotation of protective and systemic fungicides that reduce the chance of fungicide resistance developing.

·         Petroleum spray oils, milk and bicarbonates might offer some control.

Mosaic

Cause: Watermelon mosaic virus and Muskmelon yellow mosaic virus.

Mosaic

Symptoms: Light and dark green mottling of the leaves. Distortion of leaves and stunting of the plant might occur. Marrow and summer squash fruit might show sunken concentric circles or a raised marbled pattern

Control: 

·         Use tolerant varieties.

·         If only a few plants are affected they should be removed and destroyed.

·         Insect control is an option if monitoring is practiced and appropriate insecticide applications are made. However, the time for transmission by the aphids is so short that chemical control might not be viable. 

·         Plant residue should be incorporated as soon as possible after harvest.

·         White plastic mulch is used in areas where there is a history of this disease. It is used to confuse and deter aphids from visiting and feeding on plants.

Damping-off of seedlings and fungal root rots

Cause: Damping-off and root rots are caused by several fungi including Pythium, Rhizoctonia and Fusarium

Damping-off of seedling

Symptoms: The first symptoms are water-soaked lesions occurring at soil level. This leads to wilting and seedling death. Often, plants that have survived damping-off might show symptoms of root rot. Roots can have a watery Grey appearance, particularly the finer feeder roots.

Favored by: Cool temperatures, high soil moisture and poor aeration. Root rots of older plants are also common in moist hot conditions, especially under plastic mulches.

Control:

·         Usually sporadic outbreaks and can be difficult to control.

·         Fungicide seed treatments provide some control of damping-off.

·         Plant to allow vigorous growth, and do not over water. 

·         Ensure residue from previous crops is broken down.

·         Careful irrigation management is important with drip under plastic.