Whiteflies go through 6 life stages, egg, 1st instar, 2nd instar, 3rd instar, 4th instar or pupae and adult. Adult whitefly and eggs are generally observed on the underside of leaves on the upper quarter or so of the plant, while 3rd instars and pupae are found mainly on lower leaves. The Silverleaf whitefly can be a bit tricky, because adults can be found anywhere on the plant. Whitefly females lay between 100 and 400 eggs each depending on the crop. The 1st-3rd instar and adult stages are the ones that feed on plant tissue and are, therefore, the ones that cause an issue in the greenhouse.

Fungal diseases are common in many greenhouse crops and can cause huge losses in production if left untreated. Treating preventatively or at the first signs of disease will help prevent production loss. There are some biofungicides that are currently available, though the most common way to deal with fungal diseases is chemical application

Biology

There are many different kinds of fungi that negatively affect greenhouse crops, many of which have very different life cycles. Despite this, the majority of fungi go through a stage where they produce spores. These spores are how the fungi reproduce and spread throughout the crop. Dry conditions in the greenhouse promote the production of spores, while moist conditions promote the germination of spores. Many fungal diseases are spread through the air, media and/or plant tissue.

Signs and Symptoms

Symptoms can vary widely depending on the type of fungal disease and the plant variety it is infecting. However, many cause discolouration of leaves, stem and/or roots, often times leading to wilting.

Strategies

• reduce fluctuations in humidity
• keep leaves dry and prevent guttation
• keep good air circulation
• sanitize greenhouse between crops
• do not reuse slabs and/or blocks
• use varieties that are less susceptible

Examples

botrytis, powdery mildew, downy mildew, pythium, fusarium, blight and root rots.

Bacterial diseases are less common than fungal disease, but can cause just as much damage to crop production. They are generally not as obvious as fungal disease and can take a trained eye to identify. There is no economical cure for bacterial diseases and infected plants should be removed from the greenhouse.

Biology

Bacteria are single-celled organisms that live on or in roots, leaves and stems of host and even non-host plants. They are capable of multiplying rapidly under ideal conditions and are often limited to a few plant species. They prefer high moisture and temperature conditions and spread through water, media and plant material, however they need a wound or a natural entry point, like a stomata, to enter the plant.

Signs and Symptoms

Common symptoms of bacterial diseases include wilting of entire leaves or leaf margins, yellow leaves, V-shaped lesions on leaves and grey-black and water soaked looking stems.

Strategies

• buy disease-free stock
• isolate plants for a period of time to look for signs of disease
• disinfect tools and propagation area often
• clean irrigation system after each crop

Examples

bacterial canker in tomatoes, bacterial wilt in cucumbers, Xanthomonas campestric and Ralstonia solanacearum.

Like fungal and bacterial disease, viral disease can cause a lot of damage in the crop and severely impact production. They are less common than fungal diseases and are harder to identify in early stages, like bacterial diseases. There is no way to cure viral diseases. Plants should be removed from the greenhouse to prevent further spread.

Biology

Viruses are microscopic organisms that can only multiply within a living host cell. They generally spread to other parts of the plant through the vascular system and can be spread between plants by sucking insects, like aphids, tools and by handling plant tissue. Often times entire plant families will be susceptible to the same viruses.

Signs and Symptoms

The severity of symptoms depends on how long the plant has been infected. Some common symptoms include vein banding, ring spots, dead spots, irregular or mosaic like colouration and abnormal growth. Viruses can also cause stunting of the plant or death of the growing head.

Strategies

• buy disease free stock
• isolate new plants in a completely different area of the greenhouse
• disinfect tools and propagation areas often
• minimize stress on the plants (it makes them more susceptible)

Examples

tobacco mosaic virus, tomato spotted wilt virus, cucumber mosaic virus and melon necrotic spot virus.