Virus Diseases of Grapevines

W. B. Hewitt.

A killer of grapevines, Pierce's disease has twice threatened to destroy the vineyards of California, once between 1884 and 1900 and again between 1935 and 1941. In the earlier days, the disease was known by various names; mysterious disease, vine plague, Anaheim disease, and California vine disease.

It first started taking its toll of the vineyards of southern California near Anaheim and Pomona during the growing season of 1884. It spread to other sections of Los Angeles and Orange Counties and thence into San Diego, Riverside, San Bernardino, Ventura, and Santa Barbara Counties. It destroyed more than 35,000 acres of excellent vineyards. Actual losses were more extensive than just the vineyards; local wineries had to close down.

There were no records of Pierce's disease in the San Joaquin Valley during the early epidemic. Growers recalled its first appearance in the valley near Poplar in Tulare County in 1917. It was reported in the same county near Woodlake in 1921, and in other parts of the county in 1927 and 1931. Reports of the disease had become fairly numerous by 1934, and indicated its spread into Kern and Fresno Counties. By 1935 it was increasing rapidly throughout the San Joaquin Valley. Its continued development over the next 4 years indicated that another epidemic had erupted in the center of California's table-grape and raisin industry. By 1940 the disease had developed in nearly all of the grape-growing regions of the State except the Livermore and Martinez of Alameda and Contra Costa Counties.

The incidence of disease reached a peak of nearly 6 percent of 300,000 acres of vines in 1941. Losses Losses ranged from only a few vines to the complete destruction of some vineyards and even entire vineyard districts. The disease declined rapidly after 1941 to less than one-half of one percent in 1946. Since then the number of diseased vines in the San Joaquin Valley has varied but has remained low.

The occurrence of diseased vines in individual vineyards, in districts, and in the entire San Joaquin Valley followed a rather consistent pattern. At first diseased vines were widely scattered and irregularly distributed. Later they became concentrated in more localized areas, not apparently associated with environment, soil, or cultural practices. Large numbers of diseased vines were found in vineyards adjacent to irrigated pastures and alfalfa plantings and in areas where such plantings were extensive.

In San Bernardino, Riverside, and San Diego Counties in southern California, the spread of Pierce's disease followed a pattern similar to that in the San Joaquin Valley, but the losses were relatively light. In the north-coast counties, Pierce's disease was most prevalent in vineyards adjacent to brush lands and along stream banks with abundant native cover. In some parts of the Napa Valley, the disease has continued to destroy many vines annually, though the losses have not been nearly so great as they were between 1940 and 1943.

In addition to California, Pierce's disease was found in a Thompson Seedless vineyard near Carrizo Springs, Tex., in 1941-1942. This virus is reported to be the cause of grape degeneration in Florida. A similar disease has been reported in Argentina.

The symptoms of Pierce's disease in grapevines are contingent on the variety and the locality in California. The variations, however, are in the rate and degree to which symptoms show, and e in the symptom pattern. In the hot interior valleys symptoms show earlier in the season and are usually more pronounced than in the coastal regions. Scalding and burning of the leaves, the first symptoms, may develop any time after mid-June. Leaf scalding is characterized by a sudden drying of a part of the leaf while it is still green. Tissue around the margins and the tip of the large veins dries up and later turns brown. The size of the scalded areas varies from a fraction to as much as half the leaf surface. Leaf burning, usually preceded by yellowing of the tissue before it dries and turns brown, starts about the margins and progresses, often in concentric zones, toward the base of the leaf at the point of attachment of the petiole. In newly diseased vines, these symptoms frequently show on only one cane or on the canes growing from one arm or side of the vine. As the season advances the amount of leaf burning increases. Severely affected leaves drop, leaving the petiole attached to the cane. Fruit on canes that show leaf symptoms early may be dwarfed and later wither and dry. If it is late in the season before leaf scalding occurs, the fruit may prematurely color before withering and drying.

The second and later seasons of the disease are characterized by delayed growth in the spring, followed by dwarfing of affected parts of the vines. The first four to eight leaves on the shoots growing from affected parts of diseased vines often show interveinal chlorosis or mottling and deformity. Mottling is more intense in the first leaf and becomes less with each successive one formed on a shoot. As the season advances, the leaves on these vines in the second year of disease will show scalding and burning and will drop from the vine. Some canes in severely affected vines may die back from the tips. Much of the fruit withers and dries before harvest. Canes fail to mature evenly and have irregular patches of green bark in portions that should mature to a normal brown.

Death of the root system of diseased vines follows closely the decline of the top. Roots of vines in early stages of the disease appear normal. But as the disease advances the wood tissue in the roots discolors, and they die back progressively from the tips to the vine trunk.

Diseased vines may live only a few months or as long as 5 years. Young, vigorously growing vines seldom live more than one season after they become diseased. Older and less healthy vines live longer. Diseased vines usually live longer in the cooler coastal regions than in the interior valleys.

THE CAUSE of Pierce's disease is a virus that has a wide range of hosts and is spread naturally to grapevines by several species of leafhoppers. The virus has been experimentally transmitted to healthy grapevines by grafting pieces of roots, canes, and trunks of diseased vines to healthy ones. It has also been experimentally transmitted from many other host plants to grapes by several species of sharpshooter leafhoppers. The virus does not transmit by juice transfer from diseased to healthy vines or by shears, knives, and common vineyard implements. Cuttings taken from diseased vines and infected nursery stock will carry the virus.

The incubation period before symptoms of Pierce's disease appear in grapevines after inoculation by grafting or insects is 8 weeks to 15 months, depending somewhat on the time of year inoculations were made. Young vines grown continuously in glass houses usually showed symptoms 8 to 15 weeks after inoculation. Old vines in the vineyard inoculated between May and August usually developed symptoms the same season; those inoculated in September or later rarely showed symptoms before the following July.

VECTORS OF THE VIRUS are sharpshooter leafhoppers and spittle insects; 20 species of leafhoppers and 4 species and 6 varieties of spittle insects are known to be able to transmit the virus to grapevines. Norman W. Frazier and J. H. Freitag, in experimental studies at the University of California Agricultural Experiment Station, Berkeley, found three insects, the green sharpshooter, Draeculacephala minerva; the redheaded sharpshooter, Carneocephala fulgida; and the blue-green sharpshooter, Hording circellata to be the most important vectors of the Pierce's disease virus to grapes in California. Other leafhoppers reported to be vectors are:

Carneocephala triguttata, Cuerna occidentalis, C. yuccae, Draeculacephala californica, D. noveboracensis, D. crassicornis, Friscanus friscanus, Graphocephala cythura, Helochara delta, Homalodisca liturata, Neokolla gothica, N. confluens, N. heiroglyphica, Pagaronia triunata, P. 13-punctata, P. furcate, and P. confusa.

H. H. P. Severin, also working at the California Agricultural Experiment Station in Berkeley, reported the following species of spittlebugs capable of transmitting the virus to grapevines:

Aphrophora angulata, A. permutata, Clastoptera brunnea, Philaenus leucophthalmus, P. pallidus, P. fabricii, P. marginellus, P. spumarius, and P. impressus. The grape leafhopper, Erythroneura elegantala, does not spread the virus.

The green, the redheaded, and the blue-green sharpshooters, the three most important spreaders of Pierce's disease to California grapevines, are commonly found in the vineyards. The other species of insect vectors have rarely or never been found in vineyards, but occur on various wild host plants of the virus. The green and redheaded sharpshooters are the principal vectors of the virus in the interior valleys, and the blue-green sharpshooter in the coastal areas.

Life histories, host ranges, and habits of the three important vectors were worked out by Mr. Frazier at Berkeley. The green sharpshooter is widely distributed and occurs in most of the principal grape-growing districts of the State. It frequents grassy areas of bogs, stream banks, irrigation ditches, and areas wet from faulty irrigation. Very large populations have been observed in the grass of permanent pastures in thinning stands of alfalfa that contain grasses, in young grainfields, orchard cover crops, lawns, and in the grass along roadsides and ditches. Green sharpshooters may also be found in grass in vineyards, especially areas of the vineyards where irrigation water is allowed to accumulate.

Host plants of this insect are numerous; in fact, it has been found on more than 130 species of plants, although it prefers grasses for both feeding and reproduction. Some of the common food and breeding grasses are ripgut brome, Bromus rigidus; Bermuda-grass, Cynodon dactylon; hairy crabgrass, Digitaria sanguinalis; barnyard grass, Echinochloa crusgalli; foxtail fescue, Festuca megalura; Italian ryegrass, Lolium multiflorum; darnel, L. temulentum; annual bluegrass, Poa annua; and yellow bristlegrass, Setaria glauca. Other important host plants include Johnsongrass, Holcus halepensis; toad rush, juncus bufonius; redmaids, Calandrinia menziesii; neck-weed, Veronica peregrine; and cocklebur, Xanthium canadense. Grapevines are not favored hosts of the green sharpshooter, but the insects have been observed feeding on the succulent tips of shoots and canes. The spread of Pierce's disease virus to grapevines by the green sharpshooter apparently occurs during such feeding.

The life history of the green sharpshooter includes three generations a year in the San Joaquin Valley. The insects overwinter primarily in the adult stage, but a few nymphs have also been observed to live through the winter. The overwintering adults begin to lay eggs in the latter part of February. The first-generation adults mature late in April. Second-generation adults mature about the last of June, and the third-generation adults about the first of August. The generations usually overlap and are somewhat later in the northern than in the southern part of the valley.