The warm temperatures, high humidities and crowded conditions typically employed in greenhouse production of vegetable transplants are ideal for the development and spread of a range of diseases. Fungal diseases such as damping off (Rhizoctonia and Pythium) and bacterial diseases such as black rot (Xanthamonas campestris) have the potential to kill seedlings in the greenhouse and continue to cause damage once the seedlings are transplanted in the field. The best way to prevent problems with disease during transplant production is to plant disease-free seed into a clean greenhouse. Keep seedlings with common disease problems isolated from one another in the greenhouse. Plants weakened by a lack of light or nutrient, moisture or temperature stress are more susceptible to disease. Growers should avoid irrigation practices which lead to extended periods of leaf wettness or saturation of the growth media. Early detection of any problems is critical, as greenhouse diseases spread rapidly. Infected plants should be immediately removed from the greenhouse and adjacent flats should be quarantined. Unfortunately, there are relative few disease control products registered for greenhouse use and most are only effective if applied prior to arrival of the disease. Greenhouse pests such as fungus gnats attracted by the odor produced by decaying, diseased seedlings can spread the disease to adjacent healthy plants. Growers planning to purchase transplants from a custom grower should clearly specify their management and quality expectations ahead of time and then take the time to inspect the greenhouse prior to and during the seedling production period.
Source : Greathead (2003). Prevention and management of diseases in transplanted vegetables. HortTechnology 13: 55-58.