Alfalfa Stem Nematode
Mary Olsen, Extension Plant Pathologist, University of Arizona, Tucson
The alfalfa stem nematode, Ditylenchus dispsaci, is a soilborne plant-parasitic nematode that infects alfalfa. Different races infect other hosts such as onion, oats, and strawberries, but the alfalfa race reproduces only on alfalfa in Arizona. Severe infestations of alfalfa cause stand reductions and reduced yields. Distribution in the field is usually patchy. Localized areas of infected plants may first appear as poorly developing sites a few feet in diameter then enlarge and eventually overlap, resulting in large areas of infestation. Alfalfa stem nematode is found in alfalfa-growing regions throughout the world. They are most active in cool, moist conditions. In Arizona, stem nematode has been reported in mid to higher elevations, but generally is not an important problem in the low desert areas in the summer when warm soil temperatures are inhibitory to the nematode.
Signs and Symptoms
The nematode attacks the crown bud tissues causing the buds to swell, become brittle and distorted. Stem internodes are shortened and plants are stunted. Infected plants grow back slowly after harvest, and severely infected plants may die. Infected plants are also more susceptible to winter freeze damage. A small percentage of plants may exhibit white leaves and stem, known as "white flagging", a good diagnostic tool. However, because it is rare, "white flagging" does not indicate disease severity or distribution. The nematodes can be dissected from infected crown tissue and observed under low magnification with a stereo microscope. However, there are usually many free-living non-parasitic nematodes in infection sites as well, and D. dipsaci damage may be mistaken for that of the blue aphid. Therefore, the nematodes should be identified by a nematologist.
Initial infections of D. dipsaci occur in the newly forming buds during cool, moist condition when dormant juveniles become active and after eggs hatch. The life cycle consists of male and female adults, eggs and four juvenile stages, and all stages develop within the stem tissues. The nematodes feed on the parenchymatous cells and release enzymes that cause the cells to separate, resulting in the selling of host tissue. Under optimum conditions of high moisture and 65-75oF, it completes a life cycle in 19-25 days. Females lay up to 500 eggs. D. dipsaci survives in infested plant tissue or in the soil for years in a dormant stage. It is easily moved by irrigation water, in soil carried by animals or machinery from one field to another, and in infested dry hay. It also can be seed borne.
Stem nematode is controlled by the use of resistant cultivars and cultural practices. Chemical control is not effective. Some winter-dormant varieties, such as Lahonton, are resistant. Tolerance is available in non-formant varieties such as Lew. Hot, dry weather reduces D. dipsaci acitivity. Although it is usually detectable only during January and February in Arizona, these winter infections may cause severe damage to new growth. Fields are most commonly infested by application of irrigation water that has been contaminated with run-off surface irrigation water. If possible, tail water from infested fields should not be put on other alfalfa fields. Likewise, sheep or other animals and machinery should not be moved from an infested field to a non-infested field. Rotations of at least two to three years to non-hosts such as barley, wheat, corn, cotton, and melons are needed to reduce populations in the soil.
Alfalfa, and also cotton, are unusual among the thousands of hosts of Phymatotrichum omnivorum in that both die during the first summer after planting. Alfalfa normally planted in the fall, will become infected and die during the first summer. At the lower elevations the first symptoms occur in late June or July. Circular kill patterns of varying sizes are noted. The kill patterns may be several acres in extent or they may be limited to many small patterns, less than 10 feet in diameter. The kill patterns may be restricted to certain areas in the field or scattered at random. There is no correlation between the kill patterns and soil type or low areas int eh field. Severely infested fields may be adjacent to disease-free fields. Plants initially wilt during the first hot months of summer. The fungus is inactive during winter and symptoms only occur during summer. The wilting occurs in the circular patterns that were described above. Some plants are not infected and grow normally in the kill pattern area. These plants are not resistant but are merely "escapes." Under favorable conditions over 90 percent of plants are killed in infested areas. The kill pattern enlarges form year to year as the fungus grows from infected taproots to healthy roots. The reason for varying rates of annual circle enlargement, which varies from area to area, are not understood. The disease is identified primarily by the characteristic summer kill patterns and the fact that the entire taproot is destroyed. Initially, before wilting occurs, the fungus invades and causes small lesions on the taproot. As the taproot becomes further infected, eventual wilting occurs. The fungus forms characteristics "strands" on the surface of rotted, cortical root tissue. Positive identification of the disease requires microscopic examination of the "strands" that are unique to the pathogen. Phymatotrichum omnivorum is restricted to localized areas in individual fields; it is not spread by irrigation or tillage. This is due to the fact that the fungus survival structures, strands and sclerotia, occur deep in the soil. Another characteristic of the fungus is that a fungal spore mat frequently occurs at the edge of the kill pattern during wet humid weather during late July and August.
The usual size of the spore mat is 4 to 8 inches in diameter and about 1/4 inch thick. The spore mats appear overnight. These spores are initially white in color buy become brownish in color after 2 or 3 days of growth. The powdery mass of spores produced on the surface of the mat are non-functional. They have never been germinated and they play no role in dissemination of the pathogen.
The most common symptom of downy mildew occurs on leaves. Most leaf infection is in the lower canopy of the plant because the micro-climate there is more favorable for the pathogen. Infected upper leaf surfaces are bleached in appearance. During wet and humid weather the fungus sporulates on the lower leaf surface. The sporulation appears downy and violet in color. This area coincides with the bleached-yellow tissue on the upper leaf surface. Scrapings from the sporulating area reveal, under the microscope, the unique structure of the fungus. With experience, a hand lens can be used, in the field, for identification. Defoliation, caused by leaf infection, can be extensive in susceptible cultivars during wet and cool weather.
The fungus invades stem and crown tissue. Stem lesions are irregularly shaped ranging from oval to diamond. The stem lesions are sunken and straw to black in color. The fungus in wet, hot weather produces masses of fruiting structures (acervuli) that can be seen in the field with a hand lens. Stem lesions may enlarge, girdle and kill stems. Scattered death of stems int he field is a common indicator of the disease.
The symptoms of Verticillium wilt overlap those caused by several stress factors including Phytophthora root rot, Rhizoctonia stem rot, antracnose, certain nutrient deficiencies and insect damage. The first symptoms usually appear in scatered plants during cool weather. Presumably, in Arizona the diease would appear int he second or third year plantings during the winter in our nondormant varieties. Nondormant varieties shown to be susceptible in recent studies in California include CUF101, UC-CIBOLA and Moapa 69. Leaflets become bleached and eventually dry. The taproot may show yellow to brown discoloration in the vascular system. The only positive method of identifying the disease is to isolate and identify the pathogen from infected vascular tissue.
In Arizona the effects of AMV infection on alfalfa are variable ranging from masked infection (plants are infected but show no symptoms) to mild mottle and yellowing of leaves. Symptoms are more common during the winter. High summer temperatures mask symptoms. Most infected plants never show any symptoms.