Sources of Healthier Alfalfa

Fred R. Jones, Oliver F. Smith.

Crown wart of alfalfa.

Alfalfa as a forage crop in the United States is recognized as consisting of strains of Medicago sativa and of hybrids of that species with Medicago falcata. M. sativa occupies the southern and central alfalfa regions. The apparent hybrids appear to use the superior hardiness of the M. falcata parent to give them longevity in the severe climate of the northern part of the range of the crop. Thus two species, which vary greatly within themselves, produce in their combined resources a crop that in the hands of progressive agriculturists has spread across a wide range of climate.

As often happens in such wide and intensive culture, serious diseases have appeared. Many have reduced the quantity and quality of alfalfa forage. To overcome the loss, breeding for resistance has been undertaken. Already those efforts have demonstrated that the qualities of two variable species can be utilized to develop resistance to many of the diseases.

The evaluation of the resistant characters is one of the major tasks that face pathologists and breeders. Therefore in this discussion of diseases of alfalfa, we emphasize sources of resistance.

OF THE NONPARASITIC DISEASES, winter injury is often the cause of weak growth in the spring and the subsequent unthrifty condition of many plants. Besides, injured tissues often become the avenues of entrance for parasitic micro-organisms that weaken and kill the plants. In the northern alfalfa regions, effects of winter injury can be modified by using hardy or adapted varieties. Also, if snow is expected to be a factor in winter protection, fall growth should be left to hold the snow. If a field is damaged, but is to be saved, one should avoid early cutting or grazing the following spring.

Occasionally some white spots occur around the margins of upper leaves on scattered plants. Sometimes the spotting seems to be an inherited character, which develops in plants several years old when the crown begins to decay. Sometimes it develops after winter injury. It has been produced experimentally in young plants grown in soil deficient in potash. Thus at times its occurrence is taken as an indication of potash deficiency. White spot also develops extensively in response to moisture change in irrigation, although the spotting then may develop over the entire leaf instead of only at the margins.

Yellowing and dwarfing of alfalfa is often widespread from feeding of the potato leafhopper. Yellowing may also be an indication of boron deficiency.

Upper left: Leaflets of alfalfa infected with the common leaf spot (Pseudopeziza Medicaginis). Upper right: Target spot (Pleospora herbarum) on leaves of alfalfa. Lower left: A taproot of red clover showing discoloration caused by root rot fungi. Lower right: Cankers of the northern anthracnose fungus on crimson clover.

OF THE VIRUS DISEASES, alfalfa dwarf comes first. It is known to occur only in California. It was first reported in 1931 when it was localized in Riverside and adjacent counties south of the Tehachapi Mountains. In 1952 the disease occurred over much of the San Joaquin Valley and caused a rapid thinning of second- and third-year stands in about a third of the alfalfa acreage of California.

Alfalfa dwarf is due to a virus that leafhoppers and spittlebugs carry from plant to plant. Fourteen species of leafhoppers and four species of spittle-bugs can carry it. Two species of leafhoppers (Craeculacephala minerva and Corneoce phalafulgida) are the most important vectors in the field. When the leafhoppers feed upon dwarf-diseased Plants, they take the virus into their bodies and retain it for several months, during which time they may spread the virus to healthy plants when they feed. Only leafhoppers can spread the disease from field to field.

The virus that causes alfalfa dwarf is the same virus which causes Pierce's disease of grapevines.

Alfalfa plants infected with the dwarf virus gradually lose vigor for several months. Stems are short and spindly. The leaves get smaller and often seem darker in color than leaves of healthy plants. Internally, gum forms in the water-conducting elements and the woody portion of the roots and crown becomes yellow or brown. Susceptible plants usually die 6 to 8 months after infection.

Tests conducted at the University of California indicate that no available variety of alfalfa carries any degree of resistance greater than that shown by California Common. Many varieties are much more susceptible. In fields of California Common, four or more years old, practically all plants were dead or infected with the virus; about one plant in each 2,000 square feet made a normal growth even though root symptoms showed that the plants had been diseased 1 or 2 years. Several such plants were selected and self-pollinated to produce seed. Progenies from most of the selected plants were quite tolerant to the virus and maintained normal growth two to three times as long as unselected California Common. The plants were used to develop a variety; California Common 49, highly tolerant to the dwarf virus.

WITCHES'-BROOM OF ALFALFA was first recognized in the United States in 1925. It also occurs in Canada and Australia. Except in a few localities where outbreaks were severe, it has been considered of minor importance.

The first outbreak occurred in Salt Lake County, Utah, in 1925. In some fields 60 to 65 percent of the plants were infected. Farmers used short crop rotations to combat the disease effectively.

An outbreak occurred in Methow Valley and Yakima Valley of Washington in the 1930's. When first reported, infection ranged from 25 to 60 percent. Later surveys showed that infection reached 80 percent. A similar outbreak and build-up of the disease occurred in the Nicola Valley, British Columbia, in 1943.