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  • Author or Editor: Kim E. Hummer x
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The center of diversity for white pine blister rust (WPBR) (Cronartium ribicola J.C. Fischer) most likely stretches from central Siberia east of the Ural Mountains to Asia, possibly bounded by the Himalayas to the south. The alternate hosts for WPBR, Asian five-needled pines (Pinus L.) and Ribes L. native to that region have developed WPBR resistance. Because the dispersal of C. ribicola to Europe and North America occurred within the last several hundred years, the North American five-needled white pines, Pinus subsections, Strobus and Parya, had no previous selection pressure to develop resistance. Establishment of WPBR in North American resulted when plants were transported both ways across the Atlantic Ocean. In 1705, Lord Weymouth had white pine (P. strobis L.), also called weymouth pine in Europe, seed and seedlings brought to England. These trees were planted throughout eastern Europe. In the mid-1800s, WPBR outbreaks were reported in Ribes and then in white pines in eastern Europe. The pathogen may have been brought to Europe on an infected pine from Russia. In the late 1800s American nurserymen, unaware of the European rust incidence, imported many infected white pine seedlings from France and Germany for reforestation efforts. By 1914, rust-infected white pine nursery stock was imported into Connecticut, Indiana, Massachusetts, Minnesota, New Hampshire, Ohio, Pennsylvania, Vermont, and Wisconsin, and in the Canadian provinces of Ontario, Quebec, and British Columbia. The range of WPBR is established in eastern North America and the Pacific Northwest. New infection sites in Nevada, South Dakota, New Mexico and Colorado have been observed during the 1990s.

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Powdery mildew (Sphaerotheca mors-uvae) severely infects young shoots and leaves of black currants (Ribes nigrum) and red currants (R. rubrum) in the Pacific northwestern U.S. Environmentally sound control measures are being sought as alternatives to sulfur or demethylation-inhibiting fungicides. This study examined the effect of mineral oil spray on powdery mildew infection in susceptible black and red currants. Mineral oil at 8 mL·L-1(8000 ppm) was applied to plants until runoff at 0-, 2-, and 4-week intervals from April through June in 1999 and 2000 on eight currant cultivars growing in Corvallis, Ore. Shoot and leaf surfaces were rated for powdery mildew incidence in early July both years. Oil applications significantly reduced mildew severity in vegetative growth as compared with that of the unsprayed control. The disease control from 2-week interval and 4-week interval oil applications was not significantly different.

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Powdery mildew (Sphaerotheca mors-uvae) severely infects young leaves and stems of gooseberry (Ribes uva-crispa) throughout the world. Environmentally friendly control measures are being sought as alternatives to sulfur or demethylation inhibiting fungicides. This study examined the effect of a mineral oil spray, the biological control agent Trichoderma harzianum Rifai strain T-22 (Trichoderma), a combination mineral oil + Tricoderma, and the chemical fungicide thiophanate, on powdery mildew severity in `Industry,' a susceptible gooseberry. Mineral oil at 8 mL·L-1 (1.0 fl oz/gal), Tricoderma at 4 g·L-1 (0.5 oz/gal) and thiophanate at 1.45 mL·L-1 (0.186 fl oz/gal), and mineral oil + Tricoderma mix was applied to plants until runoff at 2-week intervals from February 2002 through April 2002, on potted `Industry' plants growing in a greenhouse in U. S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository (NCGR), Corvallis, Ore. The percent of infected leaves per plant were calculated and the percent of infected stem surface areas were visually rated in mid-April. The fungicide, mineral oil, and mineral oil + Tricoderma treatment applications significantly reduced powdery mildew severity inboth leaves and stems as compared with those of the unsprayed plants. The stem powdery mildew reduction levels of the mineral oil or a combination of mineral oil + Trichoderma treatments, were not statistically different than that of thiophanate, which is reported as commercially acceptable. We recommend mineral oil spray, or mineral oil + Tricoderma, as alternatives to fungicide control of powdery mildew on leaves and stems of young gooseberry plants.

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A pawpaw (Asimina triloba) regional variety trial (PRVT) was established at the U.S. Department of Agriculture, Agricultural Research Service, National Clonal Germplasm Repository (NCGR), Corvallis, Ore., in Fall 1995. This orchard was a replicated planting of 28 commercially available varieties or advanced selections from the PawPaw Foundation (PPF; Frankfort, Ky.), with eight replicate trees of each selection grafted onto seedling rootstocks and planted in a randomized block design. Two years after planting, 32 trees had either failed to establish or had died after an initial healthy start. By July 1999, 25% of grafted trees had died due to a vascular wilt-like disease, and 2 years later mortality exceeded 50%. Grafted selections with the lowest symptom severity include 1-7-2, 2-54, 7-90, 8-58, 9-58, `Mitchell', `PA-Golden #1', `Taylor' and `Wilson'. Seedling guard trees were unaffected until July 2000, when six guard trees of 76 died and 10 more were declining. By July 2001, 14 guard trees were dead. No fungi were consistently isolated from declining trees. A number of bacteria were isolated from infected trees, but no specific pathogen has been confirmed as the causal agent. Polymerase chain reaction (PCR) tests for phytoplasmas and for the bacterium Xylella fastidiosa were also negative. Research is ongoing to determine if a bacterial pathogen was the cause of the pawpaw decline in the Oregon PRVT.

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