A walk into a fruit and vegetable garden might have you seeing spots, blotches, mildew, and spores. This week, numerous garden samples have been brought into the Extension office for identification. OSU Extension Master Gardener Volunteers have enlisted the help of the office microscope, a hand lens, the primitive white paper test, and Extension Fact Sheets and Bulletins to help in the identification and control of some plant problems that gardeners are experiencing.
Let’s do a quick review of pathology, which involves the study of plant diseases. Plant problems can be separated into two categories — living (biotic) and non-living (abiotic).
The nonliving diseases aren’t contagious and won’t spread from plant to plant. Some examples include nutrient deficiencies, growth under less than ideal light, moisture, or temperature conditions, or toxicities like that caused by the black walnut tree. Biotic diseases are caused by a living pathogen and also are known as infectious. They can move within the plant or can spread between plants. Pathogens can infect specific plant parts including the leaves, shoots, stems, crowns, roots, tubers, fruit, seeds, and vascular tissue.
Most plants are immune to most pathogens — and the gardeners cheer loudly; however all are susceptible to attack by at least one pathogen, with some being susceptible to many — and the gardeners weep.
Here are three plant problems identified this week. The first two are biotic, and the third is abiotic.
•Septoria Leaf Spot of Tomatoes. Small water-soaked spots scatter throughout the leaves, usually starting on the lower and older leaves. The small spots “grow” to become 1/16 to 1/8 inch in diameter with dark margins or edges and tan centers. If you look closely in the center of the spot, there are black fruiting bodies that resemble small pimples. Leaf drop might occur if the disease is severe. This pathogen will survive on weeds like horse nettle, Jimson weed, and nightshade. What to do? Keep the garden weed free, rotate plants from year to year, and use a fungicide to protect plants if Septoria is an annual pest problem.
•Powdery Mildew of Vine Crops (cucumbers, melons, squash, pumpkins, and ornamental gourds). Small patches of fine, white threads develop on the surface of leaves, usually in late July. The patches grow together and eventually cover stems and leaves with white powdery masses of spores. Severe infections will cause the leaves to turn yellow, wither, and die. What to do? Plant resistant cultivars whenever possible. Avoid overcrowding, watering from overhead, and extra fertilization. Once powdery mildew appears, regular fungicide applications can protect other plants from the spread of the disease through the garden.
•Blossom-End Rot of Tomato, Pepper, and Eggplant. Starts out as a small water-soaked area usually at the bottom, or blossom end of the fruit. The lesion will “grow” and become sunken and turn black and leathery. Severe cases can cover the lower half of the fruit. What to do? Maintain a soil pH around 6.5. Avoid drought stress and wide fluctuations in soil moisture by using mulches or irrigation. Provide one inch of water per week for proper growth and development.
So get out today and take a close look. My fingers are crossed that your garden is pathogen-free, but if not, hopefully you will find the disease before it has spread and your garden will keep producing all season long.
Amy Stone is an extension educator with OSU Extension Lucas County. If you have questions, call the OSU Extension Master Gardener Volunteer Horticultural Hotline at 419-578-6783. Volunteers are on hand Monday, Wednesday, and Friday from 10 a.m. to 1 p.m. Questions also may be e-mailed to firstname.lastname@example.org and possibly answered in a future Plant to Plate column.