George H. Clough

Coat protein transgenic resistance to watermelon mosaic and zucchini yellows mosaic virus in squash and cantaloupe.

Three yellow crookneck squash (Cucurbita pepo var. melopepo) and five cantaloupe (Cucumis melo, Reticulatus group) lines, genetically altered for resistance to zucchini yellow mosaic virus and watermelon mosaic virus, were field tested in 1993 and 1994, respectively. During both years, nontransgenic plants were inoculated with virus before transplanting to provide a high virus threat to the transgenic plants. Before and after transplanting, serological testing (enzyme-linked immunosorbent assay [ELISA]) was used to obtain baseline information on transformed plants and to confirm field virus infection. In both years, plant disease development was rated weekly ; yield was assessed in 1993. Disease progression, yield, and end-of-season ELISA indicated a significant reduction in disease incidence in the transgenic lines. Total squash yield did not differ between the transformed and unchanged lines, but the transgenic lines yielded more marketable fruit than did the nontransgenic line.

Comparison of Application Methods on Deposition and Redistribution of Chlorothalonil in a Potato Canopy and Potential for Control of Late Blight

The effect of application method by aircraft (fixed wing), ground, and chemigation on deposition of chlorothalonil in a potato canopy was compared over 2 years. Initial chlorothalonil deposition was greatest with ground application, less by aircraft, and least with chemigation. After application by aircraft or ground, the upper canopy had more chlorothalonil than the middle, and the lower canopy had the least. With chemigation, the upper canopy also had more chlorothalonil, but amounts in the lower and middle canopy were similar. Redistribution occurred with time as material from the upper canopy moved downward. With a 9-day application frequency using ground or aircraft methods, residue levels throughout the treated potato canopy were maintained, even if distribution of the fungicide within the canopy was poor from the initial application. With chemigation, low initial chlorothalonil levels were further decreased due to redistribution, increasing the likelihood that levels could fall below that which may be needed to adequately control late blight, particularly beyond 7 days after application.

First Report of Bacterial Fruit Blotch of Watermelon in Oregon

Most of the watermelons, Citrullus lanatus (Thunb.) Matsum. & Nakai, consumed in the Pacific Northwest during the summer months are grown in the southern Columbia Basin under dry (<5 cm rainfall), low relative humidity (46 to 57%), and high temperature (29 to 41°C) conditions, using transplants, plastic mulch, and drip irrigation. During May 1996, irregularly shaped, water-soaked lesions were observed on cotyledons and first true leaves of watermelon cv. Sangria transplants growing in a greenhouse. Similar lesions were observed later on older leaves in a commercial field of cv. Millionaire. Microscopic examination of symptomatic tissue revealed bacterial streaming, and isolation on nutrient agar consistently yielded numerous creamy to off-white bacterial colonies. Bacteria from purified, single colonies were Gram negative and rod shaped. Physiological characterization by the Biolog GN Bacterial Identification System (version 3.5) showed a similarity of 0.971 to the Biolog description for Acidovorax avenae subsp. citrulli. Pathogenicity of two strains was confirmed in three separate tests by hypodermic needle infiltration of cotyledons or by stab inoculation into hypocotyls of 12 to 24 21-day-old cv. Crimson Sweet seedlings with aqueous suspensions of bacteria containing approximately 6.0 × 108 CFU/ml. Inoculum was prepared from 48-h-old nutrient agar cultures. Test plants were incubated in the greenhouse at 21°C, under a 16-h photoperiod. Hypocotyl and cotyledon inoculations produced water-soaked lesions within 24 to 48 h on both the hypocotyl and cotyledons or just the cotyledon, respectively, on plants inoculated by either method. No symptoms developed on control plants infiltrated or stabbed with sterile water only. Isolations from three symptomatic seedlings yielded colonies similar in morphology to those used for inoculation. Tests of two purified cultures by Biolog indicated the bacteria were A. avenae subsp. citrulli. The symptomatic test plants were transplanted to fields, and the maturing melons developed large, dark green, water-soaked lesions with irregular margins. Similar fruit symptoms were seen in commercial fields. Labels on seed used in commercial production and in our tests warned of risks related to fruit blotch. This is the first report of bacterial fruit blotch of watermelon in Oregon. This disease may have a significant impact on watermelon production in the Columbia Basin.

Reduced Ethylene Synthesis and Ripening Control in Tomatoes Expressing S-Adenosylmethionine Hydrolase

Using standard Agrobacterium binary vectors, we have introduced a SAMase-encoding gene derived from E. coli bacteriophage T3 into the tomato genome. This results in transformed tomato plants that exhibit significantly reduced levels of S-adenosylmethionine (SAM), the substrate for conversion (through ACC synthase) to 1-aminocyclopropane-1-carboxylic acid (ACC) which is the first committed step in ethylene biosynthesis. Lack of a sufficient pool of SAM for conversion to ACC in fruit results in tomatoes with significantly reduced ethylene biosynthetic capabilities and a modified ripening phenotype. Typically, this phenotype is characterized by fruit in which ripening on the vine is delayed while ripening off the vine may be essentially suspended.