Farming operations will either help or hinder management of soilborne pathogens, and, to head off losses, vegetable growers must learn which pathogens they face and tailor practices accordingly, says a University of California, Davis plant pathologist.
Krishna Subbarao, who is based at the USDA Station in Salinas, outlined management principles and pointers at a recent plant disease seminar in Salinas.
He said pathogens that cause disease in vegetable crops either survive in soil or have some cycle in the soil that enables them to infect the crop to cause significant losses in yield and/or quality. They are often difficult to predict, detect, and diagnose. Problems magnify when one or more pathogens occur in a field simultaneously.
They fall into two categories: those that survive in the soil anywhere from a few months to several years and those that are transient in the soil. The first includes, he said, “the common culprits: fungi, nematodes, and viruses.”
The second group is made up of various bacteria.
Production methods, including cropping history, influence where pathogens occur in the soil. Vertically, most are found in the top 10 inches of soil, where host roots and tissues and other organic matter are found. Horizontally, pathogens survive on residue of diseased tissue where host plants have grown.
Subbarao said tillage operations break up, move, and bury crop residue with the pathogens that attach to it in the soil. While deeper tillage can spread some of them farther into the soil profile, exposure to heat, cold, and drying closer to the surface may kill others.
Soil type, texture, pH, moisture, temperature, and nutrient levels are among factors that govern how pathogens behave. Well-aerated, well-drained soils create conditions that discourage root diseases, while poorly drained soils tend to favor the survival of several, including pythium, phytophthora, fusarium, and verticillium. Only a few root diseases, such as common scab of potato, are favored by drier soils.
Soil pH factor
Soil pH has significant bearing on some soilborne pathogens. For example, clubroot on crucifers is severe in acidic soils with a pH of 5.7 or less. It has been managed in commercial broccoli fields in the Salinas Valley for years by liming the soil to raise the pH to levels of 5.7 to 6.2. The disease is virtually eliminated with pH levels greater than 7.3 to 7.4.
Some crop residues have been found to be antagonistic to certain soil pathogens.
Plant nutrition and its links to soil pathogens have been the subject of much research, and Subbarao said, in general, high nitrogen levels tend to keep a host's fleshy tissue longer in a vegetative state, making it more vulnerable to attack from pathogens. However, he added, some pathogens are predisposed to invade plants that are weakened by shortages of soil nitrogen.
Not to be overlooked is the type of nitrogen available to the crop and the resulting soil chemistry that favors pathogens. Subbarao and others have reported that the positive charge of an ammonium ion attracts it to plant roots, causing the release of hydrogen ions into the surrounding soil. As the hydrogen ions lower the soil pH, diseases that thrive in acid soil are encouraged.
On the other hand, nitrates favor diseases such as vascular wilts by altering the pathogens' virulence and increasing the susceptibility of hosts.
He said diagnosing soilborne diseases is essentially the same as for those on the plant above ground. He recommended preparing a list of known suspected soilborne pathogens, which over time can help narrow the group of pathogens to watch.
Complete examination of the entire plant will reveal all symptoms to monitor for a profile of the plant's condition.
Relating symptoms of plants having symptoms to site in the field and irrigation or other cultural practices will help determine disease distribution.
Plant samples should be analyzed by a qualified laboratory to confirm the true cause of a disorder.
Keeping a record of soilborne diseases found at each site in a field will create a historic record of pathogens. This is important for future reference in selecting crop rotations because many of the pathogens may linger for years on a site.
Moving to the management of soilborne diseases, Subbarao said the severity of a disease is determined by the pathogen's primary inoculum acting with the particular host plant and environmental conditions.
The object is to reduce the amount of inoculum needed to set off an infection in the crop. The amount needed for verticillium wilt in strawberries is quite small, just two microsclerotia per gram of soil.
Among management options are fumigation, solarization, host plant resistance, and green manure crops. Success with biological controls has been spotty. Cultural controls in combination with other options may be needed.
Subbarao said in the case of Sclerotinia minor, the cause of lettuce drop, subsurface drip irrigation can be an answer. “The more uniform moisture of furrow irrigation encourages Sclerotinia, and the drier soil with drip means less of the pathogen.”
He went on to say that his research has shown that if the soil under drip irrigation is kept dry enough about 3 centimeters around and below the plant the disease can be managed.
The most familiar soilborne diseases, including seed decay, damping off of seedlings and root and crown rots, are rots that go to subsurface tissues or vascular wilts starting with root infections. But according to Subbarao, a few soilborne pathogens cause foliar disease symptoms and damage that appear on the above ground parts.
One example is lettuce anthracnose, whose microsclerotia survive in the soil. When raindrops splash soil laden with these structures onto lettuce leaves, the fungus causes a leaf-spot disease.
In like fashion, Sclerotinia sclerotiorum bodies rest in the soil until certain environmental conditions trigger development of mushroom-like apothecia that release aerial spores. The spores then find their way to crop foliage and cause a foliar disease symptom.