Fighting Plant Disease

This is part two of a series on plant diseases that you may be afflicted with at this time of the year—but not before I mention that even horticulturists are not immune to attacks by the likes of powdery mildew.About 15 winters ago I decided that my living room windowsills might be the perfect spot to begin growing African violets. Yes, it can now be told that I’d never grown African violets, so the time was long overdue. I went to a local greenhouse and picked out a few healthy specimens, and they did just great. That was, until the day I realized that the powdery stuff on the foliage wasn’t pollen, but powdery mildew stains.

I made a panicked call to my local plant pathologist, who gave me even worse news … there wasn’t a thing I could do. It seems that the microclimate created near the cool glass of the window that was hit with the warm and humid interior air of the living room was just what the dormant mildew spores needed to have a party … and so they did. But as winter ebbed, the problem solved itself with the introduction of an open window and plenty of fresh air. Unfortunately, as soon as the humidity and temperature were just right, the violets were again doomed to mildewland.

Now, back to what may be your problems. Mosaic is a virus disease that you see most often on peppers, cucumbers and corn. The symptoms are a green and yellow mottling of the foliage and an overall stunting of the plant’s growth. Fruits may have light splotches and a warty appearance. The best solution will be next year, when you should seek those varieties that are resistant to this disease (the seed packet or plant tag will say so), but if you want to grow heirloom peppers that aren’t resistant, then you’ll need to control aphids, as again these insects are the vectors.

Late blight is a fungus disease that affects tomatoes and potatoes. The symptoms are dark, irregular spots on the leaves that are often accompanied by a white mold on the undersurface. Fruits and tubers can also be affected, especially in cool damp weather. Unfortunately there is only one sure way to control this problem, and it is with regular use of fungicides. Good sanitation also helps, especially destroying any leftover crop residues and using healthy transplants or seed potatoes next year.

Bacterial blight is a disease that affects peas and beans. Water-soaked spots develop on pods, stems and leaves. During wet weather the spots enlarge, and creamy ooze may appear. In dry weather the spots dry up. During the winter the bacteria can make a home in seed or left-behind crop residues. To control the problem, don’t use home-grown seed, rotate crops and don’t work amid wet foliage. Literature also indicates that seed produced in the western United States is much less prone to carry the problem.

Damping off is caused by a soil-borne fungi that can affect all plants. The symptoms are poor germination, rotting of seeds and decaying of lower stem and roots on seedlings (which causes them to fall over), and lesions on roots or stems. Good drainage is essential in making sure that this problem isn’t yours, and sterile soil (as in peat lite mixes) as well as clean flats and pots will help. Many seeds can be pretreated with a fungicide.

Fusarium wilt is also a fungus disease, and it affects tomatoes, potatoes, vine crops and others. It’s usually noticed as leaves begin to curl upward and wilt during the heat of the day, often recovering at night. In the later stages the plant wilts and never recovers, but don’t confuse this with the normal daytime wilting that tomatoes are prone to in the heat of the summer. Look for discoloration of a stem cutting, as this is usually a foolproof sign, and expect to see this problem mostly in hot, dry weather. There are no chemical controls available, and crop rotation seems to be of little value, but resistant varieties of seeds and plants are available.

Blossom end rot is a physiological disorder that affects tomatoes, peppers and eggplants. The typically diseased plant yields fruits that have sunken, tough, black patches at the blossom end. It was discovered that the problem is caused by a calcium deficiency coupled with uneven moisture in the soil. If you have had this problem, it is easily corrected, but for next year’s crops. Mulch your soil to keep the ground evenly moist, avoid excessive use of high-nitrogen fertilizers, and add lime if your calcium levels show up low in a soil test.

Bacterial soft rot can be extremely frustrating because of the speed of infection and nearly complete destruction of fruits. The disease affects lettuce, onions, cabbage and other fruits and veggies. It first appears as small rot spots that rapidly enlarge and become soft and watery. Within three to five days the entire fruit is worthless slime. Again there is no chemical control, and good garden sanitation as well as good air circulation and drainage will help.

Finally, if you haven’t had enough, there is gray mold. This fungus disease affects lettuce, strawberry, flowers, onions, squash, eggplants and others. The symptoms appear as dark, water-soaked spots on leaves and stems, and during humid weather (here in the Hamptons?). The fungus will appear as a gray mold on affected areas that quickly spreads to the whole fruit. The disease is a problem in cool, damp weather and can be controlled with crop rotation as well as fungicides.

So do you have the picture? If it’s hot and dry they get you one way and if it’s cool and wet they get you the other. Good reason for the home gardener to be paranoid, but in fact a gardener with some experience under his or her belt can anticipate, plan and control these diseases to such a point that in most years they are little more than occasional nuisances. For beginners, the secret is in identifying these problems, learning to plant resistant varieties and practicing good garden sanitation.

Now, it’s not all bad news. There are actually somewhat good viruses that result in some of the striations and variegations that we see in plant foliage. There are two natural enemies of the dreaded gypsy moth caterpillar. One is a virus called NPV and the other is a fungus called Entomophaga maimaiga. In very wet springs this fungus can become rampant among gypsy moth populations and decimate infestations to the point where they can virtually disappear even after several successive years of attack … as has happened out here in 2001 to 2002.

In last week’s column I promised a little treat for tomato growers. In order to take advantage of this treat you’ll need an iPhone or iPad. Go to the iTunes store and buy the $2.99 app Tomato MD. The app was co-authored by Margery Daughtrey, Dan Gilrein, Margaret McGrath and others from Cornell’s Long Island Horticultural Research Farm in Riverhead. Keep growing.