Cold Late Spring and Control of Gray Mold

Published: May 1, 2014

The consequence of a cool late spring for tomatoes in greenhouses or high tunnels is the increased incidence of Botrytis blight or Gray mold. It is a disease caused by the ubiquitous fungus Botrytis cinerea. The disease is highly favored by cool weather and it is always associated with excessive moisture and highly humid (damp) environment. In covered beds and greenhouses with poor ventilation, just the humid condition within a tomato canopy at night suffices for the disease to develop. Although the disease begins in cool weather, it could also potentially infect tomatoes in a warm temperature but at a reduced rate. In protected systems like hoop houses and greenhouses, the disease can establish and spread itself very rapidly resulting in a significant economic loss on many crops.

The best 'cure' for the disease is warm, sunny weather, which we often get, and then it can rapidly diminish. But in years like this spring and last year, the weather has persisted for the disease to worsen. Growers should consider having a chemical control product on hand and to apply it at first sign of the disease. If the weather changes so the disease lessens, then stop applying product. This is preferred to the other way around (hoping the weather will change and then having to control the disease when it becomes severe). There are five chemical control options that provide good or very good control (see Table below). Just a few years ago we only had two products and the better of the two (chlorothalonil, Bravo or Equus) only provides fair control. Understanding a bit more about the disease is important to realize that it can build up in the growing environment to more readily infect next year's crop.

As mentioned above, Botrytis cinerea requires a relatively cool temperature 64.4 – 73.3 °F (18-23°C) for best growth. Growth of this fungus is inhibited as temperatures go beyond 89.6 °F (32 °C). The fungus produces conidia that are easily windborne and hence can be easily blown from field to field. Conidia are borne on conidiophores and it is the arrangement of conidia which gives the fungus its name, botrys which means ‘a cluster, bunch of grapes’ in Greek. In addition, the fungus produces overwintering spores called sclerotia to be able to survive from season to season. Sclerotia may be formed in a plant tissue and may germinate forming mycelia that produce conidia.

Symptoms and signs: symptoms start as gray to brown discoloration, water soaked, tan, brown spots that become covered with a profuse growth of gray, velvety dusty mold (mycelium and conidia) arising from these necrotic tissues. This growth gives the diseased tissue a fuzzy, gray-brown appearance (Fig. 1). Gray mold infects all the above ground parts of a tomato plant and it is mostly associated with mature plants that have a dense canopy. Pruning of leaves and clusters provides wounds that are ideal for the initiation of gray mold (Fig. 1).

Non-chemical methods of disease control: It’s always good to remember that cool and damp weather favors the disease! The following cultural methods will be useful to prevent gray mold:

• Keep leaves dry – avoid sprinkling water, since the fungus is easily spread by splashing water and wind.
• Maintain healthy plants – follow proper fertilizer and irrigation recommendations as well as pruning practices.
• Provide good air circulation – do not overcrowd plants, remove weeds timely.
• Sanitation - is one of the best ways to reduce this disease. Collect and discard affected plant parts.
• Liming acid soils to increase calcium content of plants will reduce the susceptibility of the plants.

Disease Management using Fungicides: regardless of the type of fungicide type, sprays should start before a dense canopy of the tomato plant develops. In addition, most fungicides registered to control gray mold are protective in their action and will not suppress an established infection. Always, read the label and consult extension associates nearby your area for any updates on fungicide uses.