Bacterial and viral diseases can spread very quickly throughout a greenhouse, especially if a closed system is being used, where nutrient solution is recirculated. Although UV and ozone water purifiers can be used in hydroponic systems, they can be very expensive and have adverse effects on the minerals in the solution. If raw, incoming water is from a contaminated surface source, UV and ozone treatment may be necessary to assure pathogen-free source water.
Basic sanitation is necessary in hydroponic greenhouses. Workers’ clothing and shoes should be free from soil, in fact, some greenhouse operations have shallow trays of bleach solution for workers to clean off their shoes before entering the growing areas. Hands and tools must be cleaned regularly to prevent spread of disease within a greenhouse. Smoking or chewing tobacco must be strictly forbidden in the greenhouse, and workers should wash their hands after handling tobacco to prevent transmittance of tobacco mosaic virus (see below).
Bacterial canker (Clavibacter michiganensis): this is a seedborn disease, which is unlikely to be found in hybrid seeds purchased from a reputable dealer. The disease is first noticed on the lower leaves, which exhibit unilateral wilting on one side of the leaf, then wither and die. Often, the petioles remain on the plant, which helps distinguish bacterial canker from other diseases. Fruit may be affected with small, raised white spots that develop brown centers. Moderate temperatures and high humidity favor disease development. No cure, but spread of the disease can be controlled by sterilizing all equipment and media.
Bacterial spot (Xanthomonas vesicatoria): First symptoms: small, dark, water-soaked spots on leaves, progressing to dried, cracked lesions surrounded by yellow. Spreads rapidly via wounds, such as pruning or sucking insects. Warm, moist temperatures favor the disease. Copper sprays can provide some control. Destroy all plant residues.
Bacterial wilt (Burkholderia solanacerum, also called Pseudomonas solanacearum and Ralstonia solancearum): Symptoms begin as wilting of lower leaves, followed by whole plant, without characteristic yellowing caused by other diseases. A quick diagnosis: place a freshly cut infected stem in a glass of water, and a white, milky stream of bacterial ooze will be visible flowing from the cut. Highly contagious, this disease can cause serious damage. No effective controls known.
TMV photo by M. Jensen
Tobacco mosaic virus (ToMV) will cause disfigurement of the leaves and stunted growth. Sucking insects or hands and tools of workers in the greenhouse can transmit the virus. Prevent the disease by using resistant cultivars and making sure all workers wash hands with soap and water after handling any tobacco products.
Tomato yellow leaf curl virus (TYLCV): a gemini virus transmitted by the tobacco whitefly, Bemisia tabaci, serious problem in the Middle East. Causes leaf edges to curl upward, and interveinal chlorosis. Fruit set is greatly reduced. Particularly destructive if young plants are attacked, as very little fruit will be produced. Currently, new, resistant cultivars are being developed. Meanwhile, complete control of whitefly is the best protection.
Other mosaic viruses, such as common mosaic, Aucuba mosaic, and cucumber mosaic can all infect tomatoes. Symptoms include mottled areas of the leaves, yellowing, and stunted growth. Prevention includes sanitation, especially the removal of any weeds that may also be susceptible to these diseases. Aphids can transmit viruses, so control of insect pests will reduce the risk of viral problems.
Bemisia transmitted gemini virus on tomato (as yet unidentified). This can be a severe virus on tomatoes growing in the southern latitudes of the United States and in Mexico. The symptoms are often similar to tomato yellow leaf curl virus and the fruit can exhibit off-colored ripening. Again, the best control of this disease is to control the whitefly which spreads it.