By Eric Sideman, Ph.D., Director of Technical Services, MOFGA
Tunnels and greenhouses are now being used widely to produce early and often blemish free tomatoes. However, high humidity is difficult to avoid under plastic, and it creates an ideal environment for fungal diseases that can spread very quickly and cause widespread damage. Two common diseases of tomatoes in tunnels are gray mold and white mold (also called stem rot), which are caused by two fungus species, Botrytis cinerea and Sclerotina sclerotiorum, respectfully. The similarities and differences in the biology of the pathogens is interesting, and an understanding will help growers control problems.
The fungus can be seen first as a bit of white mold in the axils or stem joints, but is too often not noted until it is much further along and whole stems begin to yellow and then turn a bleached light gray. Sclerotia form inside the fungus mat inside the stem. Sclerotia are small, hard, black, tightly compressed bits of fungus that are resting bodies, i.e., the sclerotia are the means by which the fungus rests between seasons. The sclerotia fall to the soil and lie there over the winter. When conditions warm up again, they will send up fruiting bodies that release thousands of airborne spores. The sclerotia in the top inch of soil are primarily responsible for the inoculum for the primary infection.
The spores require dead tissue to initiate infection. Usually that tissue is something like a fallen petal that lands in the crotch of a stem. Relatively long periods of wetness (16 to 72 hours) are needed for spore germination. After sufficient fungal growth has taken place on the senescent tissue, the healthy stem is invaded. Some plant-to-plant spread of the disease occurs by growth of the fungus, but most of the infection is from the initial release of spores from the sclerotia.
This fungus is ubiquitous and affects hundreds of species of plants. During the growing season if there is humid air, it is probably carrying Botrytis spores. The mature growth of the disease is a characteristic fuzzy, gray felt on the plant tissue, including stem, leaf and fruit. Clouds of spores are released when the plant is shaken, and more infection takes place. The earlier growth is much harder to spot and includes pale areas on leaves, girdled stems and ghost spots on fruit.
The initial infections of plants in the spring come from spores released from sclerotia, similar to white mold. However, most crop infection, unlike white mold, is from secondary spores blowing around all the time from other infections. The disease usually starts on senescent foliage, very commonly the sepals and dying petals. After the inoculum increases on senescent tissue, young living tissue is invaded. Prolonged wet periods are not necessary for spread of the disease, but some humidity is. The humid conditions within the tomato canopy at night and the leaf wetness from condensation are enough for quick disease spread. The disease begins developing under cool conditions and spreads under warm conditions.
1) The first lines of defense for both of these diseases are air movement around plants, ventilation and controlling humidity and condensation. Development of both fungi requires high humidity. Efforts to reduce relative humidity pay off. Condensation of moisture occurring on the inner layer of plastic is a big problem. The water dripping off the plastic will contact the spores on the plant tissue and cause them to germinate. Four to six hours of wetness at 40 to 65 degrees is ideal for germination and growth. The dripless plastics on the market can help.
The most effective way to reduce humidity is to dilute the greenhouse air with outside air of lesser humidity. This is particularly useful in the early morning when the house has been closed and has been getting humid over the night from condensation. Cracking the vents and turning on the heat will work well, because the warmed air will hold more moisture. During the day keep the air moving, either with natural ventilation or fans in order to avoid humid air collecting in the plant canopy.
2) Give plants enough space. This is important for two reasons. First, it allows more air movement and thus helps reduce areas of trapped, humid air. Also, white mold sometimes spreads by plant-to-plant contact.
3) Inspect plants frequently and remove and destroy dead and dying plant material. Remember, this is important because these fungi most easily colonize senescent tissue.
4) Remove any diseased plant parts, because these are sources of gray mold spores and also overwintering sclerotia.
5) Maintain adequate calcium in the soil, but pay attention to the balance with phosphorus. Resistance of plants to fungal pathogens has been associated with high calcium nutrition – presumably because a calcium complex forms with the pectic substances, which renders the cell walls resistant to hydrolysis by fungal enzymes. In both field and lab experiments, gray mold decreased on tomato plants as calcium content increased. The lowest incidence was on plants with high calcium and low phosphorus contents, while the greatest disease incidence was on plants with low calcium and high phosphorus contents. The balance was important, since plants with higher amounts of both calcium and phosphorus had similar disease incidence as plants with low amounts of both calcium and phosphorus. Excess phosphorus in plants may prevent calcium from forming in pectic substances. And, excess phosphorus in the soil will reduce the calcium availability to the plant. A petiole tissue analysis showing a calcium to phosphorus ratio of 2:1 is best.
6) Biocontrol is a possibility for white mold. Recent work has reported on a fungus that attacks the sclerotia. This could well work, because the primary infection by Sclerotinia is from the spores released from the sclerotia. Unfortunately, even though Botryitis also overwinters as a sclerotia, the spores from the wide range of other hosts outside of your fields are so numerous that killing the local sclerotia will have little effect.
The biocontrol is based on an organism called Coniothyrium minitans, which is a mycoparasite, i.e., it parasitizes fungi. This is a well documented sclerotial parasite of many Sclerotinia species, with numerous reports of successful biocontrol of Sclerotinia diseases.
A new commercial product on the market called Contans W.G. is applied as a soil amendment three to six weeks prior to planting. This provides sufficient time for the fungus to contact and parasitize sclerotia in the soil and reduce the amount of spores released for crop infection. I have not seen the product for sale yet, but I have talked to the company that distributes it in the United States (it is made in Germany). David Goulet, at Encore Technologies, said that EPA labeling was recently received, and he will be glad to talk with farm product supply companies. So, if you would like to try it, get your supplier contact from Encore (www.encoretechllc.com) at 952-404-9596.
You can contact Eric with your questions about organic farming and gardening at [email protected].