Leaf mold of tomato
Leaf mold is not normally a problem in field-grown tomatoes in northern climates, but it can cause losses in tomatoes grown in greenhouses or high tunnels due to the higher humidity found in these environments. Foliage is often the only part of the plant infected and will cause infected leaves to wither and die, indirectly affecting yield. In severe cases, blossoms and fruit can also be infected directly reducing yield.
Host and pathogen
Leaf mold is caused by the fungus Passalora fulva (previously called Fulvia fulva or Cladosporium fulvum). It is not known to be pathogenic on any plant other than tomato.
Signs and symptoms
- The oldest leaves are infected first.
- Pale greenish-yellow spots, usually less than 1/4 inch, with no definite margins, form on upper sides of leaves
- Olive-green to brown velvety mold forms on the lower leaf surface below leaf spots.
- Leaf spots grow together and turn brown. Leaves wither and die but often remain attached to the plant.
- Infected blossoms turn black and fall off.
- Fruit infections start as a smooth black irregular area on the stem end of the fruit. As the disease progresses, the infected area becomes sunken, dry and leathery
- Optimal growth at relative humidity greater than 85%
- Optimal temperature is between 71 °F and 75 °F, but disease can occur at temperatures as low as 50 °F and as high as 90 °F
Biology and disease cycle
Spores of P. fulva can survive for 6 months to a year above ground at room temperature. At this time it is unknown if spores will survive on the surface of stakes, tools, and high tunnel walls from one season to the next in Minnesota's climate. The pathogen forms dark, hard resting structures within infected plant debris. These structures will produce an abundance of new spores when exposed to air and are the most likely means for P. fulva to survive from one season to the next. The leaf mold pathogen can survive on and in tomato seed and may be introduced to an area by this route.
Spores of P. fulva can start an infection at a wide range of temperatures. Although some disease can occur at humidity less than 85 %, relative humidity at or above 85 % will favor severe leaf mold epidemics. Infection often starts on lower leaves. New spores form on the lower surface of infected leaves within 10 to 12 days. If humidity remains over 85%, these spores will infect new leaves. Within the growing season, multiple generations of the pathogen can be completed, with conidia being spread from leaf to leaf and plant to plant by wind, rain/overhead irrigation, tools, workers and perhaps insects.
Although varieties designated as resistant to leaf mold can be found in many seed catalogs, these may or may not be effective in reducing disease in Minnesota. There are currently 12 known races of P. fulva. Each resistant cultivar protects against only one to a few races. It is currently unknown which races of P. fulva exist in the United States of America. Therefore it is unknown which varieties will effectively reduce disease. In addition, the population of P. fulva on a farm can change if new races emerge or are introduced from another area. Growers with a history of leaf mold are encouraged to try resistant varieties on a small scale to determine their efficacy at the location. Resistant varieties should be used in combination with cultural control practices as part of an integrated disease management program.
- Use drip irrigation and avoid watering foliage.
- Space plants to provide good air movement between rows and individual plants.
- Stake, string, or prune to increase airflow in and around the plant.
- Circulate air in greenhouses or tunnels with vents and fans and by rolling up high tunnel sides to reduce humidity around plants. Keep night temperatures in greenhouses higher than outside temperatures to avoid dew formation on the foliage.
- Remove crop residue at the end of the season and either burn it or bury it away from tomato production areas.
- Sterilize stakes, ties, trellises etc. with 10% household bleach or commercial sanitizer
- Clean the high tunnel or greenhouse walls and benches at the end of the season with a commercial sanitizer.
Below is a partial list of fungicides available for management of leaf mold on tomato. Applications should be made prior to infection when environmental conditions favor disease to be the most effective. The first leaf mold infections of the season have been observed in the first week of June in Minnesota high tunnel tomatoes. Fungicide applications should be repeated according to label instructions. It is important to alternate between different chemical families to avoid the development of pathogen insensitivity to particular active ingredients.
Note: In Minnesota a high tunnel is considered a greenhouse for the sake of pesticide application. Read all label instructions carefully prior to use. If the product label prohibits use in a greenhouse, it cannot be legally applied in a high tunnel. If the product label provides specific instructions for use within a greenhouse or does not mention use in a greenhouse, it can be used in a high tunnel. Different formulations of the same product may vary on greenhouse use. The instructions on the label attached to the pesticide container must be followed.
|Active ingredient||Common product names||Chemical family||Comments|
|Difenoconazole and Cyprodinil||Inspire Super||3 and 9, respectively||Good|
|Difenoconazole and Mandipropamid||Revus Top||3 and 40, respectively||Good|
|Cymoxanil and Famoxadone||Tanos||27 and 11, respectively||Good|
|Copper||Kocide, Champ Formula 2, Nu-Cop 50DF||M||Fair|
|Mancozeb||Dithane, Manzate, Penncozeb||M||Fair|
|Mancozeb and Zoxamide||Gavel||M and 22, respectively||Fair|
|Azoxystrobin and Difenoconazole||Quadris Top*||11 and 3, respectively|
* While Quadris Top is often registered for greenhouse use, other Quadris formulations are not. Please ensure that the formulation of Quadris that you purchase is labeled for greenhouse use.
The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by the University of Minnesota Extension. A pesticide label is a legal document. Always follow the pesticide label directions attached to the pesticide container you are using. The site of use or plant to which the pesticide is to be applied must be listed on the label or the pesticide cannot be used. If a pesticide label prohibits use within a greenhouse, it cannot be used in a greenhouse or high tunnel in Minnesota. Remember, the label is the law.