One of the main benefits to growing tomatoes in high tunnels or greenhouses is that the plants avoid many fungal diseases that plague field-grown tomatoes, most notably Septoria leaf spot (Septoria lycopersici) and early blight (Alternaria solani). Protected environments can present their own challenges, however, and there are pathogens that thrive in these environments.
Most high tunnel tomato growers are probably familiar with leaf mold (Fulvia fulvum). This disease is very common in high tunnel conditions. It can best be managed by reducing humidity and increasing airflow, and by using resistant varieties. There are many naturally-occurring genes in tomato that confer complete resistance to specific races of leaf mold; most varieties have one or more of these genes. For example, in recent UNH trials, the varieties Geronimo, Imperial 643, Massada, Panzer and Rebelski remained free of symptoms in all years in which they were grown, despite high levels of disease on susceptible varieties in the same tunnel.
Another pathogen that is less common, but perhaps more damaging to a tomato crop, is powdery mildew. Here in NH, we saw several cases of powdery mildew in high tunnels in 2012, and we have seen a couple of cases already thus far in 2015. In contrast to leaf mold, powdery mildew can spread more quickly and cause more total leaf loss, resulting in greater crop losses. Further, powdery mildew cannot be easily managed by using resistant varieties. While some varieties appear to develop symptoms more slowly than others, most cultivated varieties are ultimately highly susceptible.
There are over 700 species of powdery mildew pathogens that infect over 10,000 host species. Tomato powdery mildew is caused by three species: Oidium neolycopersici first caused epidemics in European greenhouse tomato crops in the 1980s, Leveillula taurica has been found in field grown tomato in the western U.S.(and has a very broad host range), and Oidium lycopersici has been reported only in Australia. Based on plant diagnostic lab reports, O. neolycopersici is likely to be the species that sporadically occurs in New England greenhouses and high tunnels. While this species can apparently infect other members of the Solanaceae (including Datura, Nicotiana, Petunia, and eggplant), the Cucurbitaceae, and many other families, the National Plant Diagnostic Lab network only has reports of it infecting tomato and Plectranthus, an ornamental foliage plant.
Identifying the symptoms of powdery mildew vs. leaf mold
Leaf mold starts as distinct yellow regions on the upper surface of middle-age or older leaves. Usually, brown sporulation can be seen on the undersides of these distinct yellow spots. If the disease progresses, the spots can coalesce and can eventually kill the leaf.
Powdery mildew starts as diffuse light green or yellow regions on the leaf, followed by powdery-looking white spores on the upper (and occasionally the lower) surface of the leaf. These start in small spots but quickly spread until the entire leaf surface is white. The leaf then dries out and becomes dead and crispy.
Once leaves have been killed, it can be hard to tell for sure which disease(s) are present. It is easiest to identify symptoms on leaves that have just recently been infected. If there is any question, digital photos can be sent or physical samples can be brought to the closest plant diagnostic lab.
Contrasting leaf mold and powdery mildew
The powdery mildew pathogens are obligate parasites, which means that they do not survive unless they are on a living host plant. This means that a fallow period without a susceptible host will successfully break the disease cycle. Year-round production systems do not experience this fallow period, and are therefore probably most at risk of maintaining powdery mildew inoculum for new crops.
Unfortunately, powdery mildew thrives and produces spores at the same temperatures at which tomato thrives (60-77F). The asexual spores, or conidia, do not require leaf wetness to germinate, which makes them well adapted to greenhouse or tunnel production.
There are several organic and conventional fungicides that are registered for control of powdery mildews in greenhouse or tunnel tomato production. Many of these products are effective if used when symptoms are first observed, but most require repeated applications to maintain control. See the New England Vegetable Management Guide, greenhouse tomato section for specific recommendations.
Cheryl Smith, UNH Plant Health Specialist
Meg McGrath, Powdery mildew of tomato. http://www.longislandhort.cornell.edu/vegpath/photos/powderymildew_tomato.htm
Ales Lebeda et al., 2014. Resistance mechanisms of wild tomato germplasm to infection of Oidium neolycopersici. Eur. J. Plant Pathol. 138:569-596.
J.B. Jones et al., 1991. Compendium of Tomato Diseases, APS Press.
N.D. Warren et al., 2015. Performance of high tunnel tomato cultivars in Northern New England. HortTechnology 25(1):139-146.