Gray fuzzy sporulation characteristic of leaf mold on the underside of a tomato leaf. Yellow chlorotic spots will be visible on the upper leaf surface.
Leaf mold is a disease caused by the fungal pathogen Fulvia fulva. It general only affects the foliage first starting on the older leaves. The initial symptoms are pale green or yellowish spots first noticeable on the upper leaf surface which eventually turn a more distinct yellow with undefined margins. The fungal sporulation that develops on the underside of the leaf is olive green in color and tends to be more dense and darker in color towards the center of the lesion. The pathogen does not affect the fruit but reduces overall photosynthesis and can lead to defoliation reducing fruit quality and quantity.
Yellow chlorotic lesions on the upper leaf surface.
Leaf mold on tomato appears to be an ongoing problem not just in high tunnels but it also has been reported in open field production this season. This disease is very dependent on high relative humidity greater than 85% so the frequent rains, fog and dew have created favorable conditions in many parts of Pennsylvania. If you had problems this year or have had problems managing this disease in the past especially in a high tunnel, in the future consider planting less susceptible or resistant cultivars.
Similar to the resistance found in late blight cultivars, tomato breeders have identified several resistance genes (named Cf which stands for Cladosporium fulvum, the oldest name of the pathogen; the current name is Passalora fulva, recently changed from Fulvia fulva) in various wild species of tomato that have been conventionally breed into several commercial tomato cultivars. In total, nine resistance genes have been identified. Cf-9 is the most common resistance gene that has been bred into commercial tomato cultivars and it confers resistance to all five races of the leaf mold pathogen. In seed catalogs it is indicated by a number of different abbreviations so it is important to pay attention to what diseases the abbreviations stand for in the tables and cultivar descriptions.
Implementing cultural practices that maximize air circulation around the plant will greatly aid in disease management. Protectant type fungicides such as mancozeb or Catamaran (chlorothalonil plus potassium phosphite) and can be used to help manage the disease during the season. Products like Scala (pyrimethanil), Revus Top (mandipropamid plus difenoconazole), and Quadris Top (axozystrobin plus difenoconazole) and are also options but later two need to be applied with the sides of the high tunnel are rolled up. See the Commercial Vegetable Production Recommendations for a list of additional products and rates.
In research trials conducted in NY, several cultivars including Primo Red, Red Deuce, Red Mountain, Geronimo or Panzer have been described as being less susceptible or resistant based on performance under high tunnel conditions. However this season, there have been multiple reports of leaf mold on Red Deuce. Unfortunately from the previous list, only Panzer contains the resistance gene(s) for leaf mold however it has also performed will in high tunnel trials conducted by Steve Bogash, former Horticulture Educator in Cumberland County. Other possible fresh market or salad type cultivars with resistance include Caramba (Seminis), Clermon (Syngenta, Johnny's), Pink Cupcake (Sakata), Poseidon (Seminis), Rally (Enza Zaden), Rebelski (Johnny's) and Trust (Johnny's). As you try new cultivars, it is important to keep good notes of their performance in terms of yield, quality and disease susceptibility. Pathogens are always changing so if you see symptoms of diseases like leaf mold or late blight on resistant cultivars please contact your local Penn State Horticulture educator.