Vegetable Production Guide

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Disease Control

This section was updated - 16 July 2021

Bacterial Canker (Clavibacter)

Symptoms may begin as wilting of leaves at the lower and outer parts of the plant. The edges of the leaves turn brown and yellow and curl upward. Light coloured streaks may develop on the stems, shoots and leaf stalks. Later, cracks may develop in the streaks. The stem pith becomes brown, dry and mealy. Plants sometimes die but they usually produce fruit. Sunburn can be severe because of the loss of outer, protective leaves. Diagnostic white “birds-eye” spots are found on green fruit. On ripe fruit, spots have a raised, tan-coloured, rough area about 3 mm across and a surrounding white halo.

The most likely source of disease is contaminated seed and transplants, but bacteria may survive in the field on plant debris.


  1. Use hot-water-treated seed (see Planting section of this guide). Seed treatment with diluted hydrochloric acid or household bleach diluted 1:10 in clean tapwater has also been used.
  2. Disinfect used seedling trays prior to use (see Planting section of this guide).
  3. Use sterilized soil in flats and seed-beds.
  4. Destroy old vines immediately after harvest.
  5. Follow a two or three year crop rotation.

Bacterial Speck (Pseudomonas)

Numerous, tiny, dark brown to black specks of less than 1.5 mm diameter appear on the surface of green or ripe fruit. Larger, dark brown to black spots may occur on the leaves and stems. Fruit are only infected when green. This disease is more common in cool, wet weather.


  1. Use hot-water-treated seed (see Planting section of this guide).
  2. Sow treated seed in sterilized soil.
  3. Disinfect used seedling trays prior to use (see Planting section of this guide).
  4. Kocide, applied for bacterial spot, may also reduce bacterial speck if applied early when green fruit are developing. See Table 3, Tomato Disease Control.

Bacterial Spot (Xanthomonos)

Leaf symptoms are similar to bacterial speck. Small dark specks appear on the green fruit, and later enlarge to become brown scabby spots. These spots are crater-like, and may be surrounded by a greasy looking white halo.

See , Table 3 (Tomato Disease Control) and controls for bacterial speck.

Botrytis Rot

Symptoms are gray mold rot of foliage, stem and fruit. Pale green or silvery ring spots with a brownish puncture in the centre, known as ghost spot, occur on green fruit. The same rings are generally yellow on ripe fruit. This disease is associated with wet, humid weather.

See Table 3, Tomato Disease Control. Captan applied for anthracnose may also control Botrytis.

Foliage Blights and Fruit Rots

Early Blight (Alternaria)
Leaf spots are dark brown to black and have concentric rings or zones within the spot. Black spots may develop on stems. On the fruit, large black leathery sunken areas are produced. This disease is common in the Interior.

Late Blight (Phytophthora)
Irregular greenish-black water soaked areas appear on older leaves. Brownish cankers are found on stems. Fruit rot may also occur. This disease is more common under cool, moist conditions. The same fungus also causes late blight of potato. When starting tomatoes in the greenhouse, keep the foliage dry and space the plants to allow good air circulation around them. After planting out in the coastal area, it will then be necessary to spray every 5 – 10 days with fixed copper or other protectant fungicide. If the weather is wet or unsettled, spray every 5 days. Later, when the weather dries up, the interval can be increased to every 7 – 10 days. Separate fields of tomatoes and potatoes as much as possible as the spores will spread from one to the other. Do not use Ridomil/MZ, Bravo/Ridomil on tomatoes as it will result in illegal residues. These products are registered for control of late blight on potatoes only. Clean up diseased crop refuse and remove it from the field in the fall if feasible. Failure to do so may result in the fungus overwintering in the field.


This is a disease of ripe fruit. When spots first appear they are small, sunken and circular. They can increase in size to 1 cm and are usually found on the side of the fruit next to the soil. Older spots are depressed with concentric rings, a tan centre and numerous dark specks.

A regular spray program is required to avoid losses due to the two blights and anthracnose fruit rot in the late summer and early fall when wet weather begins. Begin spraying in late June and repeat every 7 to 10 days depending on weather conditions. Use one of the products in Table 3, Tomato Disease Control.

Mosaic and Streak (Virus Diseases)

Tobacco Mosaic: Pale mottling of leaves and sometimes of fruit. Alfalfa mosaic causes brown streaks and distortion of fruit.

Streak: Brown stripes on stem, brown blotches on foliage and brown irregularly shaped pits on fruit.

Mosaic is much more common than streak. Tobacco mosaic is spread from one tomato crop to another on seed, clothing, soil, hands and tools. Alfalfa mosaic is spread only by aphids.

Control: Tobacco or common mosaic:

  1. Destroy virus of tobacco mosaic on the seed coat a day or two before planting by soaking the seed in a solution of tri-sodium phosphate at 100 g/ L of water for 20 minutes. This treatment is only needed if seed has not previously been treated with hot water (see “Bacterial Canker,” this section). Spread out to dry in a warm room. Use seed treated with Thiram. Rogue out affected plants if any disease appears.
  2. Tobacco mosaic can persist many months on clothing stored in the dark, but not if hung in the light. Contaminated clothing can be a source of disease for a new crop, and only clean clothes should be worn while working with plants, especially young ones.
  3. Tomato debris overwintering in the soil is the main source of tobacco mosaic in new crops, so old vines should be removed immediately after harvesting is completed. Steaming soil in the greenhouse will control mosaic, but chemical sterilants will not.
  4. Handling is the most frequent way of spreading tobacco mosaic throughout a crop. Wash hands in a solution of tri-sodium phosphate at 33 g/ L of water and then scrub with soap and water after smoking or handling a diseased crop. Because tobacco may carry this disease, avoid smoking in the greenhouse area. Infection on tools is best removed by either boiling them in water or by steam sterilizing them. Dipping tools in tri-sodium phosphate solution at 33 g/L of water can be used but is less effective.

It has been found that the spread of mosaic from diseased to healthy plants and infection from outside sources can be reduced sharply by the use of a milk spray. From 4 to 24 hours before seedlings are to be handled (pricked out), they should be watered thoroughly, then sprayed with natural whole or skimmed milk or reconstituted powdered milk, 3.0 L/10 L of water. The milk is allowed to dry on the leaves. It gives effective protection for 24 hours. Twenty-two litres of milk will spray approximately enough plants to set out in 1 hectare.

A second milk spray should be applied to the plants just prior to transplanting.

Tomato Spotted Wilt Virus

This disease is occasionally seen in field tomatoes as a result of planting infected seedlings. Infected tomato plants may not grow well and have bronzed leaves and yellow ring patterns on fruit. See Pest Management section of this guide for control options.

Verticillium Wilt

The causal fungus is soil or seed-borne. First, lower leaves wilt and drop, then petioles turn yellow and wither and, finally, the entire plant may wilt.


  1. Disinfect used seedling trays (see Planting section of this guide).
  2. Grow seedlings in virgin or sterilized soil.
  3. Use seed treated with Thiram seed protectant.
  4. Follow four-year rotation plan, avoiding crops susceptible to Verticillium wilt (e.g. potatoes, melons, peppers and eggplants).
  5. Choose soil that is well drained and not underlain by hardpan.
  6. Do not plant out until soil temperature has reached 14°C or, if possible, 16°C.
  7. Avoid overwatering and maintain a high humus content in the soil.
  8. Select resistant varieties.

Root Rot

RootShield WP (Trichoderma harzianum Rifai strain KRL-AG2) is a biological fungicide for the suppression of Root Rot in greenhouse vegetable transplants.  When applied to transplants, greenhouse planting mix or soil, the product grows into plant roots as they develop to protect roots  against Pythium, Rhizoctonia and Fusarium. 


  1. Suspend RootShield WP using a spray volume of 30 to 45 g/100 L and apply at the rate of 55 to 110 g per cubic metre (loose) of greenhouse potting mix, soil or planting beds. 
  2. Apply using hand-held backpack or ground spray equipment. 
  3. Becomes active when soil temperatures are above 10 degrees C and is not effective while soils remain cold.
  4. More effective at suppression of root rot in neutral or acidic soils.  
  5. Should be used within 12 months of the date of manufacture.  
  6. Contains the allergens wheat and sulfites.

RootShield WP is used in organics.  It is OMRI-USA listed.  OMRI-USA = Organic Materials Review Institute of USA.  Check with your certification body before using in an organic operation.

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