Bacillus carotovorus.

Zygospore formation. Section through an acervulus.

The last two have been used by several workers in crosses with commercial varieties, and promising lines have been selected.

A high degree of resistance to gall formation by nematodes (Meloidogyne species) was found in L. peruvianum. This species is a difficult one to cross with commercial varieties. Several crosses have been made, however, and lines having a high degree of resistance have been selected.

Resistance to bacterial wilt (caused by Pseudomonas solanacearum) has been reported.

One line of currant tomato is reported to have mature plant resistance to southern blight (caused by Sclerotium rolfsii).

WATERMELON ANTHRACNOSE, caused by Colletotrichum lagenarium, is perhaps the most destructive disease of watermelons, attacking both the foliage and the fruit. It is particularly serious in the Southern States. Congo, released in 1949, is somewhat resistant. Its resistance was obtained from an African melon. Fairfax, introduced in 1952, is the first shipping variety available to southern growers which offers combined resistance to anthracnose and to fusarium wilt. The resistance to anthracnose came from the same source as Congo and the resistance to fusarium wilt from Leesburg and Hawkesbury.

DOWNY MILDEW is caused by Pseudoperonospora cubensis. A source of resistance to it was found in native watermelons of poor quality from the Dominican Republic. They were crossed to varieties resistant to fusarium wilt and anthracnose.

FUSARIUM WILT is caused by Fusarium oxysporum f. niveum. Since the release of Conqueror in 1911 there have been developed many resistant varieties that are adapted to the principal watermelon-growing areas. Among them are Improved Kleckley No. 6 and Stone Mountain No. 5 for the Midwest; Leesburg, Hawkesbury, Blacklee, Georgia Wilt-Resistant, Miles, Missouri, Queen, and Iron-sides for the South; and Klondike R-7, Baby Klondike, and Blue Ribbon for the West.

MANY SERIOUS DISEASES remain for which resistance is needed: Common blight of bean, bean and pea root rots, ascochyta blight of pea, aster yellows and mosaic of lettuce, and curly top and late blight of tomato, to mention only a few.

Often resistance has been found in a related species, but it has not been satisfactorily transferred into a good horticultural type. A higher degree of disease resistance is needed in many of our so-called resistant varieties. Recently the moderate type of resistance to fusarium wilt in tomato has been increased to near immunity; many of the newer potato varieties are more resistant to late blight than the earlier resistant introductions; most of the yellows-resistant cabbage varieties now have the type of resistance which is effective at both high and low soil temperatures.

New diseases and strains of old diseases will appear as cropping becomes more intensive and as vegetables are grown in new areas. New sources of resistance will be needed.

To meet the future problems, continuous research by plant breeders, pathologists, and other biologists is needed. There will be no time to rest on past laurels.

H. REX THOMAS, a graduate of the University of California, has been a plant pathologist in the division of vegetable crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering since 1937.

W. J. ZAUMEYER, a graduate of the University of Wisconsin, joined the Department of Agriculture in 1928. He is a pathologist in charge of bean and pea disease investigations in the division of vegetable crops and diseases of the Bureau of Plant Industry, Soils, and Agricultural Engineering.