Top tomato pathogens

Hydroponic tomatoes are one of the most popular crops grown in a greenhouse. While insufficient light, inappropriate air temperatures and mineral nutrient deficiencies or toxicities can negatively impact yield, pathogens can pose real challenges if left undetected and identified. The goal of this article is to highlight some of the most common fungal, bacterial and viral diseases that can infest hydroponic tomatoes grown in greenhouses.

Fungi

Grey mold or Botrytis (botrytis cinerea) is the most common disease in greenhouse tomato production. The most striking symptom is the growth of the fungus itself, which, as the common name implies, is a grey mold. Once infected, flowers, fruits and foliage start to decay. This fungus is ubiquitous in greenhouses, including those with hydroponic tomato crops. One of the first steps to take in managing botrytis is to remove senescing or dead plant material, as these tissues are very susceptible to infection. Additionally, reducing humidity is key in managing botrytis, and venting in the evening with the heat running is an effective way to do this.

Powdery mildew (Leveillula taurica and Oidium lycopersicum) is another common fungal disease. The name comes from the conspicuous whitish, powdery-looking growth that are spores of the pathogen. These develop on the top and/or bottom sides of foliage, depending on which species is present. As foliage coverage increases from the pathogen, yield and quality decline. Although the moisture requirement for powdery mildew to thrive is not as great as botrytis, using a similar humidity reduction strategy can help suppress the disease.

While not a bona fide fungus, pythium (Pythium aphanidermatum) is another fungal-like disease that can negatively affect tomato growth and development. One of the most common symptoms begins with root tips turning brown, followed by browning (and death) of more of the root. This disease can spread rapidly in production systems using recirculating irrigation, as the recycled nutrient solution supports the survival and spread of this pathogen. However, nutrient solutions can also be treated with UV light or ozone or does with sanitizing chemical to reduce pythium populations before being delivered to crops in the greenhouse.

Bacteria

Bacterial canker (Clavibacter michiganensis) is one of the most problematic greenhouse bacterial diseases. It can spread on the surface of tomato plants, as well as systemically within plants. Infected plants may have wilting, curling or browning leaves where the plant is infected. The disease can be further diagnosed by cutting open plants to look for stained vascular tissue and slime production, characteristic of infected plants. This disease can live on infected debris, so rouge out any plants and get them off the growing premises as well as the plants adjacent to them, as they may be asymptomatic carriers of the diseases. Tools can also harbor the disease, so sanitize them to minimize the risk of transmitting it to healthy plants.

Bacterial speck (Pseudomonas syringae) and bacterial spot (Xanthomonas spp.) are two bacterial diseases that have similar symptomology; both produce black, necrotic lesions. Diagnosing the diseases is easier on fruits, where they are more easily distinguished from one another. First, bacterial speck lesions tend to be smaller (~1/16 inch) compared to bacterial spot (~1/8 to 1/4 inch). The lesions caused by bacterial spot can also have a greasier appearance than those caused by bacterial speck. Finally, bacterial speck lesions may be surrounded by a yellowish “halo.” Free moisture can promote the spread of these diseases, so minimize the humidity and potential for condensation on plant surfaces; additionally, minimize or eliminate high-volume chemical applications to the crop to minimize free water.

Viruses

Tobacco mosaic virus (TMV) is one of the most common viral pathogens affecting greenhouse tomato crops. As the name implies, a mosaic pattern of light and dark coloration can appear on leaves. Additionally, leaves may appear deformed and overall plant growth stunted. Infected fruits may have chlorotic rings and/or ripen unevenly. It is transmitted mechanically among plants; if infected plants are handled and/or pruned, hands and/or tools must be sanitized before coming into contact with uninfected plants to limit transmission. Additionally, using tobacco can infect workers’ hands. As a result, they must be thoroughly cleaned after using any tobacco products before plants are handled. Seeds may also be infected, so purchase them from a reliable source.

Another common greenhouse virus is tomato spotted wilt virus (TSWV). This is closely related to the ornamental plant virus impatiens necrotic spot virus (INSV). This virus is easy to identify, as it causes conspicuous lesions shaped like concentric rings. In addition to reducing fruit growth, it can also render fruits unmarketable. Tomato spotted wilt is transmitted primarily through thrips (Frankliniella occidentalis and other species), so controlling their populations is key to managing this virus.

For plant viruses, prevention is key. There are no remedies for plants infected with viruses. Using practices that promote sanitation from seeding through harvest is key, and when plants are infected, rouging out and destroying infected plants are the only solutions to the problem.

The take-home message

Hydroponically grown tomatoes can be susceptible to a range of pathogens, so knowing which may be the most likely to appear in a greenhouse and detecting their presence early is essential to avoiding major problems during production.

The author (ccurrey@iastate.edu) is an Associate Professor in the Department of Horticulture at Iowa State University.