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Wes Deuel and Sven Svenson

Seedmeal (MSM) and screenings (MS) of meadowfoam (Limnanthes alba H.) were evaluated for their influence on the development of clubroot caused by Plasmodiophora brassicae in potted seedlings. Treatments included MSM at 0%, 5%, 10%, and 20% (by vol.); MSM at 10% (by vol.) plus an 8 oz. application of 3% H2O2 per pot; and MS at 10% (by vol.) pre-sowing incorporation into potting media (Sunshine Mix #1) with a 10% (by vol.) clay-loam soil known to be infested with P. brassicae resting spores. One-hundred percent Sunshine mix #1 was used as a control medium. Following media preparation, seeds of Chinese mustard (Brassica chinensis) and cauliflower (Brassica oleracea var. botrytis `Snowball Y Improved') were sown every 7 days for 4 weeks. Symptoms of P. brassicae infection (clubbing or rotting of roots) occurred in 70% to 90% of all plants grown in pots with media containing infested soil and no MSM or MS, with disease severity ranging from <25% to >50% of root systems clubbed or rotted. Chinese mustard seedlings had more clubbing compared to cauliflower seedlings. All plants grown in media containing MSM or MS showed no clubbing or rotting. Plants grown in 20% MSM or 10% MSM plus a 3% H2O2 had symptoms of phytotoxicity. Plants grown in 10% MSM or 10% MS were taller compared to controls. Although plants grown in MSM and MS showed no clubroot symptoms, asymptotic presence of pathogen has not been excluded.

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Roni Cohen*, Yosef Burger, and Menahem Edelstein

The use of grafted vegetables as one of the alternatives to soil disinfestation with methyl bromide is increasing in Israel. Watermelon (Citrullus lanatus) and melon (Cucumis melo) plants are grafted mainly onto Cucurbita rootstocks for lessening losses due to soil-borne pathogens. The contribution of the rootstock to the grafted plant's resistance depends on the nature of the disease. In general, damage caused by non-specific root-rot pathogens such as Rhizoctonia solani, Macrophomina phaseolina, Monosporascus cannonballus, and Pythium spp. are effectively reduced by using Cucurbita rootstocks. However, these rootstocks provide only partial protection from vascular diseases such as fusarium wilt, in which case better protection can be achieved by grafting susceptible melons onto monogenic fusarium-resistant melon rootstocks. The performance of the grafted plants depends not only on the rootstock but also on the scion response to pathogens and on the effect of the environment on disease development. The response of grafted and non-grafted melons of different cultivars to sudden wilt disease caused by the fungus Monosporascus cannonballus was evaluated in field trials conducted in the fall and spring growing seasons. Significant differences in disease incidence were found among cultivars, between grafted and non-grafted plants, and between seasons. Grafting reduced plant mortality in the spring and fall experiments but prevention of yield losses was more effective in the spring. More emphasis should be given to finding suitable rootstocks and adjusting agrotechniques for successful commercial cultivation of grafted melons in the fall.

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Steve Rose and Zamir K. Punja

Eighteen cucumber (Cucumis sativus L.) cultivars (long English type) were screened for their susceptibility to fusarium root and stem rot caused by Fusarium oxysporum Schlechtend.: Fr. f.sp. radicis-cucumerinum D.J. Vakalounakis using seedlings at the third true-leaf stage. Roots were trimmed and dipped into a spore suspension (105 spores/mL) of the pathogen and the plants were re-potted. A disease severity index (DSI) was used to assess disease responses 4 or 8 weeks later based on plant mortality and the height of surviving plants compared to the noninoculated controls. `Sienna', `Amazing' and `Dominica' were most susceptible to infection and the resulting DSI values were significantly (P ≤ 0.05) higher compared to noninoculated control plants. The cultivars `Korinda', `Euphoria' and `Aviance' displayed significantly lower DSI values which were not significantly different from noninoculated control plants. The remaining 12 cultivars displayed DSI values which were intermediate between the above two classes of responses. The results from this study indicate there is the potential to identify and develop cultivars and breeding lines of greenhouse cucumbers with enhanced resistance to fusarium root and stem rot.

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K.E. Maloney, W.F. Wilcox, and J.C. Sanford

`Titan' red raspberry (Rubis idaeus L.), highly susceptible to root rot caused by Phytophthora fragariae Hickman var. rubi Wilcox & Duncan (syns. P. erythroseptica Pethyb., “highly pathogenic” P. megasperma Drechs.), was planted in June 1990 in a silt loam naturally infested with the pathogen. Raked beds (0.36 m high) dramatically reduced disease incidence and severity relative to flatbed treatments. In contrast, metalaxyl at 372 mg·m-1 of row provided little benefit when applied to flat beds and provided consistently moderate but statistically insignificant effects when applied to raised beds. Relative to the flat bed system, primocane vigor was increased in 1992 by 16%, 190%, and 224% in the flat bed plus metalaxyl, raised bed, and raised bed plus metalaxyl treatments, respectively; total yields were increased by 7%, 231%, and 272% with these same respective treatment. The results indicate that raised-bed planting systems can provide substantial control of phytophthora root rot of red raspberries even when highly susceptible varieties are grown on otherwise marginal sites. Metalaxyl appears more effective as a supplement rather than substitute for raised beds under such conditions. Chemical name used: N- (2,6-dimethylphenyl) -N- (methoxyacetyl)alanine methyl ester (metalaxyl).

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Wesley T. Watson*, David N. Appel, Michael A. Arnold, Charles M. Kenerley, and James L. Starr

Phymatotrichopsis omnivora (Duggar) Hennebert (syn. Phymatotrichum omnivorum Duggar) is a recalcitrant soilborne pathogen that causes serious root rot problems on numerous plant species in the southwestern United States and northern Mexico. Apple trees [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf. (syn. M. domestica Borkh. non Poir.)] are highly susceptible to P. omnivora with most tree death occurring in the summer months. Studies were conducted from 1996 to 1999 to examine when and at what rate infection and colonization of roots of apple trees by P. omnivora actually occurs. In three-year-old trees growing in orchard soils in 45-gallon containers (171,457 cm3) and inoculated with sclerotia in August 1997, infection occurred in the nursery after 12 weeks. For trees inoculated with sclerotia in February 1998, infection occurred within 15 weeks. After 18 weeks, 100% of trees were infected after inoculation in August and 80% of trees were infected after the February inoculation. This information is vital to understanding the epidemiology of Phymatotrichum root rot in apple orchards.

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Kristin A. Schneider and James D. Kelly

We recognize the assistance of L.P. Hart for the initial isolation of the pathogen used in this study. This research was supported by grant DAN 1310-G-SS-6008-00, and the U.S. Dept. of Agriculture National Needs Fellowship program. The cost

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T.G. Beckman and P.L. Pusey

Armillaria root rot is the second leading cause of peach tree mortality (after peach tree short life) in the southeastern United States. Currently, there are no commercially available rootstocks for peach with proven resistance to this pathogen in the United States. Since 1983, we have been screening rootstock candidates for resistance to Armillaria utilizing naturally infected field sites. Inoculation of peach [Prunus persica (L.) Batsch], plum (P. cerasifera J.F. Ehrh., P. munsoniana F.W. Wight & Hedr., P. salicina Lindl. or P. angustifolia Marsh.) × peach and plum × plum hybrid rootstocks with infected plant tissue (such as acorns, Quercus sp.) prior to planting has provided a significantly increased infection and mortality of candidate rootstock lines in comparison with sole reliance on natural inoculum on an infested site.

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Amnon Koren* and Menahem Edelstein

Grafting technology for vegetable transplants was introduced to Israel eight years ago by Hishtil Nurseries, Inc. The main goal of grafting was to find a substitute for methyl bromide, the elimination of which was pending. The use of grafted watermelon transplants soon followed. Presently, more than 40% of watermelon transplants are grafted. The chief reason for the success of grafted transplants is their tolerance to soil-borne pathogens, including Fusarium, Monosporascus, and Macrophomina. Yields of grafted transplants are often much higher, and it has been shown possible to grow watermelons with saline water (4.5). A limitation of grafted transplants is that presently, we do not have a good solution for nematodes. A drawback is that in order to get good watermelon taste and flavour, the grower needs the experience to adjust agrotechniques, especially determining the best harvest date. Grafted tomato transplants were also introduced early on. Grafted tomato transplants can have excellent resistance to fusarium crown rot, corky root, and other soil-borne pathogens. Some rootstocks have been observed to tolerate water salinity of 8 ec and still produce commercially acceptable yields. Limitations to the use of grafted tomato transplants are the lack of compatibility of some of the cultivars with the rootstocks and the breakdown of nematode resistance at high soil temperatures. Melons, eggplants, and cucumbers are grafted under some conditions.

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Miriam Fishell and Laura Pottorff

Research was conducted on the fungus-like organism Pythium to observe its sensitivity to the fungicide Subdue MAXX, active ingredient mefanoxam. Pythium is a plant pathogen that causes root and crown rot in plants that are exposed to extensive soil moisture and poor drainage. This study, conducted on Pythium isolated from Colorado greenhouse crops, began in Apr. 2004. Symptomatic tissues from bedding plants and perennials were placed on selective media (P10VP). Resulting isolations of Pythium were transferred to cornmeal agar amended with the recommended low and high label rates of mefanoxam. Mycelia growth was monitored for 72 hours. Pythium sp. showing 20% growth on amended mefanoxam agars, compared to the control growth, were considered resistant to mefanoxam. Results from this study showed that about 64% of the Pythium sp. isolated from greenhouse crops in Colorado are resistant to mefanoxam. Data compiled from greenhouse integrated pest management surveys in Colorado show a high dependence on the use of mefanoxam. Research and screening for mefanoxam-resistant Pythium sp. will continue to provide Colorado growers with effective control solutions for this pathogen.

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K.E. Maloney, M.P. Pritts, W.F. Wilcox, and M.E. Sorrells

Phytophthora is a severe root rot disease in most raspberry production regions throughout the world. Disease control options are limited to raised bed culture and fungicide applications. Few Phytophthora-resistant varieties are available that have commercial quality. Little is known about how soil amendments (i.e., composts, fertilizers, and limestones) influence Phytophthora control in raspberry. We evaluated the effects of preplant soil modification on the incidence of Phytophthora root rot in red raspberries. The experiment was conducted simultaneously at two sites to differentiate between the nutritional value of the amendments and the disease control value. One site has a known history of Phytophthora and a the second site is assumed to be free of the causal organism. Raspberry plant growth and fruit yield measurements were taken for all treatments. Preplant soil application of Gypsum (CaSo4) and post-plant applications of phosphorous acid sprays (H3PO3) had the greatest fruit yields compared to all other treatments in the Phytophthora-infested site. Gypsum-treated plots had greater cane diameter, cane height, and cane density compared to the control plots on the Phytophthorainfested site. A second experiment was conducted to further investigate the use of gypsum for control of Phytophthora in raspberries. Field soil was collected for use as potting medium from each of the aforementioned sites and pathogen free `Titan' plants were established in the greenhouse. After subsequent floodings, gypsum-treated soils delayed foliar disease symptoms compared to the control plots. At the end of the experiment, the control plants had 100% foliar disease symptoms and gypsum-treated pots had 33% disease symptoms. This study suggests that gypsum could be used in an integrated approach to Phytophthora management in raspberries. Future research should identify minimal effective rates of gypsum, examine other calcium sources, and determine effectiveness in other fruit crops.