Early in the gardening year we were plagued with rabbits, squirrels, birds and other animals munching on our plants. Now when everything is lush and beautiful, other gardening problems begin to show up.
The tomato plants in my neighbor's topsy-turvy planters are loaded with fruit, but she is quite disappointed to see that many of them are afflicted with blossom end rot. Blossom end rot is a serious disorder of tomatoes, peppers and eggplants that is not caused by an insect or disease. It is the result of a physiological disorder due to low calcium content in the fruit. The tissues break down, as the rapidly growing fruit does not have enough calcium for normal growth. It usually begins as a small water soaked area at the blossom end of the fruit when it is green or ripening. It gradually becomes black, sunken and leathery. The affected area is tan on peppers and may be mistaken for sunscald.
Prevention is the only remedy for this problem. The soil pH should be 6.5 for optimal calcium uptake. Composted manure or bone meal will supply calcium, but it must be applied prior to planting. Fertilizers that have ammoniacal nitrogen can reduce calcium uptake by the roots, so use nitrate nitrogen and don't overfertilize. Avoid damaging the roots with deep cultivation. Stabilize the soil moisture level with mulches to prevent drought stress and over-watering fluctuations. The plants need one inch of water a week, but watch potted plants carefully.
Once the blossom end rot has begun, it is hard to fix the problem. Although foliar applications of calcium have little value, some gardeners report that a foliar application of magnesium (Epsom salts) can help with calcium uptake. Reduce stress on the plant by removing the affected fruit.
Plant diseases are harder to diagnose as compared to insect problems. Most insects are easy to see and you suspect them when you see holes in the leaves. Diseases are caused by bacteria, fungi or viruses and are microscopic in size. Disease symptoms often mimic excessive heat or cold, sunscald, nutritional deficiencies and low light levels.
Most diseases are caused by fungi, which cause wilting, rots or blights. The infected plants may appear chlorotic (yellow) or have other discolorations. Fungi have reproductive spores that can live in the soil for years. Plant pathogens can be spread by blowing in the wind or are carried in splashes of water. Animals, insects, tools and gardeners can move them from plant to plant.
Some plants are immune or resistant to certain diseases, while other plants are particularly susceptible to a disease. Garden phlox are especially susceptible to fungi (mildew) and some of mine have rusty or yellow leaves right now. Some plants have been bred to resist their common infections. For example, tomato varieties are labeled with V, F and or N, meaning verticillium-, fusarium-, and root knot nematode-resistant.
Diseases occur when three elements are present: a susceptible plant, the disease-causing agent (pathogen) and favorable environmental conditions. Prevention is the solution. Start with healthy transplants; plant them in well-drained, fertile soil and pick resistant varieties. Water at the base of plants rather than overhead or water in the morning so that the leaves have time to dry before evening. Increase air circulation by allowing space between plants and pruning. In the case of my phlox, I should have thinned out all but a few stems when they were just emerging. Avoid stress on plants by planting them in the correct sun exposure.
Crop rotation is important in reducing disease problems, although in home gardens tilling the soil spreads infected soil around. This is another instance where raised beds are beneficial. Plants of one family, which are susceptible to similar diseases, are rotated with plants of another family. Tomatoes, potatoes, eggplant and peppers are all in the Solanaceae family. Cabbage, cauliflower, broccoli, mustard, radishes, turnips and even sweet alyssum are in the Cruciferae family. Dill and coriander are in the same family (Umbilliferae) as carrots and celery.
Control the sources of disease with good sanitation. Clean up crop residue and remove infected plants, including the roots. Clean tools and implements as well as your hands and even shoes. Avoid moving from plant to plant when they are wet.
If your soil has been infected with a pathogen, it is possible to kill the organism by 'solarization.' Remove all vegetation, rake the soil and soak it with water. Cover the area with a clear plastic tarp or two layers of four mil thick plastic sheeting. Bury the edges 6 inches to get a good seal. Leave the plastic on for 8 weeks in the hottest part of summer.
Unfortunately, sprays or dusts must be applied before plants show infection or at their first sign to form a protective shield. There is no cure once the disease is present, so remove all infected leaves to curb the spread. A common recipe for powdery mildew is one-teaspoon baking soda, one drop of liquid soap and two quarts water. Sulfur spray or dust is used to control fungal diseases and copper sprays (Bordeaux mix) are available for bacterial diseases. Use other chemicals as a last resort, as fungicides are some of the most toxic environmental agents.