Verticillium wilt is caused by the fungus Verticillium albo-atrum, which is present in the soil. Dr. Wilhelm found that the organism tolerates both heavy clay loams and sandy loams and can exist either in an alkaline soil with a reaction of pH 8.5 or one as acid as PH 4.5.

Certain clones of the wild Pacific coast trailing blackberry, R. ursinus, and the commercial dewberry varieties, Boysen, Young, and Nectar, are highly susceptible to infection by Verticillium. Varieties of black and red raspberries also become infected with the organism. The late Dr. S. M. Zeller, of Oregon State College, however, discovered that in the latter group the Cuthbert variety is tolerant of the disease even though infected. Other resistant or immune species and varieties include clones of the wild trailing blackberry, Logan, Mammoth, Chehalem, Himalaya, and the Cutleaf Evergreen.

Once established in a field verticilliurn wilt is hard to eradicate or control. Susceptible crops should not be planted in Verticillium-infested soil. Only disease-free stock should be used in establishing new fields and any plants that subsequently show the wilt symptoms should be removed promptly with the root systems and burned. That is practical only when fewer than 5 percent of the plants are diseased.

Blackberries are also subject to other disorders of fungus origin. One, leaf and cane spot, caused by Septoria rubi, is mostly present on the dewberries in the Willamette Valley of Oregon and in parts of Idaho. Sometimes it is serious in the commercial berry-growing regions of coastal California. The fungus spores are spread from diseased to healthy tissue primarily by splashing water.

A major symptom on the leaves is small, light-colored spots, bordered with red or purple, about one-eighth inch in diameter. The spots also are present on the canes and may become so numerous as to cause premature defoliation and death of the fruiting canes. The plants are not killed but continue to produce new shoots, which become infected later in the season. The overwintering lesions on the canes become brown. In Oregon a spray of lime-sulfur in February or March is recommended for control. In California a second application with zineb is required in the early-blossom stage.

A closely related and serious disease is found on Cutleaf Evergreen blackberries in western Washington. Only the fruiting canes show symptoms of small, black, circular spots, which begin to appear in December. They enlarge, several lesions coalesce, and it is not unusual to find the base of the canes discolored black for several feet. The fruiting canes become girdled, and by March the entire cane growth may be killed. The new spring shoots show no symptoms until the following winter. In Washington the best control is a spray application with ferbam or Phygon-XL in mid-June or early July. At that time the young shoots are trellised, usually beneath the fruiting canes, and the spray thoroughly covers this new growth, protecting it from infection.

The Cutleaf Evergreen blackberry in the Pacific Northwest is also affected with cane and leaf rust caused by Kuehneola uredinis. George W. Fischer and I found the disease widespread in western Washington in 1949. Cane and leaf rust makes its first appearance in late spring or early summer, when large, lemon-yellow lesions are produced. They split open the bark of the overwintered canes. Later in the summer, minute, yellow pustules are present on the lower leaf surfaces and sometimes on the fruit. In autumn the leaf pustules become buff-colored from the formation of another spore form. How the rust overwinters has not been determined definitely. Susceptible varieties include also the Broadleaf form, two unnamed hybrids, and Chehalem. Some clones of the wild trailing blackberry, R. ursinus, are also infected. Nine commonly grown varieties of dewberry, besides Himalaya, and two numbered hybrids were found to be immune. No definite control program has been developed for the disease.

Stamen blight, Hapalosphaeria deformans, another disease found mainly in the Pacific Northwest, can cause serious losses to the fruit of Boysen, Young, and Cutleaf Evergreen blackberries. Occasionally it has been found on R. ursinus. Stamen blight is most easily detected when the flowers open. Then the anthers of the blooms are transformed into gray, moldy masses of spores. The stigmas, which are not infected, function normally; when some are pollinized by insects with pollen from healthy blossoms, a deformed fruit results with only a few normal drupelets. It is not known definitely when infection takes place, but it likely occurs during early spring from spores produced in the flowers the previous year and carried over winter in the buds. No conclusive control program has been developed, but a lime-sulfur solution with water sprayed on the canes in August has resulted in about 60 percent control.

Two bacterial diseases of blackberries are important in the West cane gall and crown gall, caused by Agrobacterium rubi and A. tumefaciens, respectively. In symptoms they are somewhat alike with the appearance of rough, warty outgrowths. In the former the enlargements are primarily present on the canes, which are split open. The knobby outgrowths in crown gall are confined mostly in the crowns and on the roots. A. rubi infects plants of the genus Rubus; while A. tumefaciens also infects fruit trees, vegetables, and ornamentals.

Experiments by L. C. Coleman, of the Dominion Laboratory of Plant Pathology, Saanichton, B. C., showed that the horsebean, Vicia faba, could be used as a differential host, for it is susceptible to infection by the cane gall organism but not by the crown gall bacterium. Severe infection devitalizes the plants.

The organisms readily contaminate the soil in which they can persist without susceptible plants for several years. All blackberries and dewberries are susceptible to infection by both organisms, as are black and red raspberries, but raspberries appear to have more resistance than the other brambles.

Pruning out and burning infected canes as soon as symptoms appear offers some degree of control. New plantings should be established on noncontaminated soil with disease-free stock. Introducing soil from a contaminated field into noninfested areas by cultivation implements or other means should be avoided.

DWARF is the most important virus disease in the trailing blackberries. It generally is present in the three Pacific Coast States and is important to the Loganberry industry.

Plants affected with dwarf take on a yellowed hue. The canes are much shorter and three or more buds are produced in the leaf axils where only one is normally present. The canes are spindly at first and in later years become stout, stiff, and unnaturally upright. The leaves are mottled with irregular bronzed and light-green splotches and are smaller than healthy. Considerable leaf distortion by crinkling and puckering of the tissue between the veins is noticeable.

The Phenomenal and Logan varieties are most susceptible to dwarf, although the thornless Logan is reported somewhat tolerant to the disease in California. The wild Rubus ursinus nus is infected and serves as a reservoir from which the virus can spread to commercial fields. The aphid Capitophorus tetrarhodus is a vector of the virus in Oregon, but the means whereby the spread is effected in California is unknown. Dwarf has not been found occurring naturally in the Boysen, Nectar, and Young varieties.

An unusual malformation of the Thornless Logan, referred to as purple stunt, has been observed in western Oregon by E. K. Vaughan, of Oregon State College. The disease is minor, but it deserves attention because if it should become generally established in the Logan fields great losses may result. Purple stunt is characterized by severe stunting. The canes seldom reach a length of more than a few inches and remain purple throughout the season. Death of affected plants is the usual result.

As with most virus diseases in other crop plants, there is no control once a plant has become infected. If stunt is not too prevalent in a field, diseased plants should be rouged out and burned.

Disease-free stock should be used when establishing a new planting. Control of the insect carriers is an important and effective method for reducing spread of viruses in the fields.

FOLKE JOHNSON is a plant pathologist at the Western Washington Experiment Station of the State College of Washington. He was reared in the Pacific Northwest. Dr. Johnson is a graduate of that institution; before returning there in 1943, he spent 4 years investigating virus diseases of plants at the Rockefeller Institute for Medical Research and Ohio State University.