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Tomohiro Okada and Yoh-ichi Matsubara

both biotic factors ( Knaflewski et al., 2008 ; Wong and Jeffries, 2006 ) and abiotic factors ( Lake et al., 1993 ; Miller et al., 1991 ; Yong, 1984 ). As biotic factors, the most common phenomenon is fusarium crown and root rot caused by Foa

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Benjamin K. Hoover and R.M. Bates

drained soils, PRR is the limiting factor in fraser fir production and is the only serious disease affecting fraser firs in Pennsylvania. Multiple species of Phytophthora are known to contribute to root rot in fraser fir ( Benson et al., 1976 ; Kuhlman

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Benjamin K. Hoover and R.M. Bates

.M. Sellmer, J.C. Despot, D.A. 2004 Postharvest characteristics of canaan fir and fraser fir Christmas trees HortScience 39 7 1674 1676 10.21273/HORTSCI.39.7.1674 Benson, D.M. Grand, L.F. Suggs, E.G. 1976 Root rot of fraser fir caused by Phytophthora

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David R. Bryla and Robert G. Linderman

Root rot is a prevalent disease of highbush blueberry ( Vaccinium spp.). Symptoms include reduced plant vigor, yellowing and premature reddening of leaves, and early defoliation of green stems ( Cline and Schilder, 2006 ). Affected plants are

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Stephen L. Krebs and Michael D. Wilson

Fifty-seven rhododendron cultivars (genus Rhododendron L.) were screened for resistance to root rot caused by Phytophthora cinnamomi, using two levels of inoculum. While a majority (77%) of genotypes was susceptible, six cultivars had moderate resistance, and seven cultivars exhibited a high level of resistance to the disease. In these resistant groupings, the severity of root rot did not increase significantly with a 3-fold increase in inoculum. Comparisons of micropropagated and conventionally propagated plants revealed no significant difference in root rot ratings. The species R. keiskei was identified as a possible source of resistance to P. cinnamomi in two of the rhododendron cultivars.

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Zhanao Deng, Brent K. Harbaugh, Rick O. Kelly, Teresa Seijo, and Robert J. McGovern

Caladiums (Caladium ×hortulanum) are popular ornamental plants widely grown for their bright colorful leaves. Pythium root rot, caused by Pythium myriotylum, is one of the few soil-borne diseases in caladium that dramatically reduces plant growth, aesthetic value, and tuber yield. Information on the reaction of caladium cultivars to P. myriotylum is not available, but would be valuable for integrated control of this disease and for breeding new resistant cultivars. Three Pythium isolates obtained from decaying roots of plants collected from a field production site and two greenhouses were evaluated for pathogenicity and potential use in experiments to screen commercial caladium cultivars for resistance. All three isolates were found to be highly virulent; they were able to cause obvious root rotting within 3 to 5 days and severe root rotting and leaf losses on susceptible cultivars within 10 days after inoculation. Nineteen major commercial cultivars were evaluated for their resistance to these isolates. Fifteen of the cultivars were susceptible or highly susceptible to Pythium infection. Four widely grown cultivars, `Candidum', `Candidum Jr.', `Frieda Hemple', and `White Christmas', were found to have a moderate level of resistance (partial resistance) to pythium root rot. Pythium infection also caused leaf discoloration, epinasty, wilting, and collapse. Regression analyses revealed a linear relationship between the root rot and leaf loss severity on Pythium-inoculated plants.

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Chrislyn A. Drake* and James F. Hancock

Black root rot is a serious disease of strawberry (Fragaria ×ananassa Duch.) that causes the death of feeder roots, the degradation and blackening of structural roots, and an overall decrease in plant vigor and productivity. The causal organisms of black root rot are Rhizoctonia fragariae, Pythium sp. and Pratylenchus penetrans (the root lesion nematode). Each organism alone can cause extensive damage to strawberry roots, but studies have shown that black root rot may be more severe when all organisms are present, indicating there is an interaction between the fungal organisms and the nematode. The current method of control for black root rot is methyl-bromide fumigation; however, methyl bromide is to be phased out by 2005, and it is not very effective in perennial matted-row systems. The objectives of the study are to measure levels of tolerance to black root rot in 21 strawberry genotypes. The genotypes were planted in four blocks each of methyl-bromide fumigated and non-fumigated soil, and were evaluated for crown, runner, and inflorescence number; yield; average berry weight; and root health. `Cavendish', `Kent', `Midway' and `Winona' showed the highest degree of tolerance, while `Jewel', `Mesabi', and LH50-4 (a F. virginiana genotype) were the poorest performers.

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Zhanao Deng*, Brent K. Harbaugh, Rick Kelly, Teresa Seijo, and Robert J. McGovern

Caladiums (Caladium × hortulanum) are widely grown for their bright colorful leaves. Pythium root rot, caused primarily by P. myriotylum, is one of the most important diseases in caladiums. This disease can dramatically reduce plant growth, impact plant aesthetical value, and lower tuber yield. Pythium infection in the roots may also lead to subsequent entry of Fusarium into tubers resulting in tuber rot. There has been a strong interest in the tuber production and greenhouse plant production industries to identify cultivars that are resistant or tolerant to Pythium. However, few studies have been conducted since the pathogen was identified, and little information is available regarding the existence of any possible resistance in commercial cultivars. Pythium isolates were made from diseased plants collected from different sites; their pathogenicity was confirmed using tissue culture-derived plants. Procedures were developed for oogonia spore production, inoculation, and disease severity assessment. Nineteen major commercial cultivars were inoculated at two spore densities and then maintained in greenhouses under growing conditions favorable for root rotting. Plant appearance, leaf characteristics and severity of root rotting were evaluated 2-3 times after inoculation. Observations indicated that the isolates were highly virulent. They induced visible root rot within 3-5 days, and caused a complete loss of the root system and plant death for some cultivars within 2-3 weeks after inoculation. Several cultivars, including `Candidum' and `Frieda Hemple' which are widely grown cultivars, had much less root rot, higher plant survival, and seemed to have moderate levels of resistance.

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Desmond R. Layne, Guido Schnabel, Kerik Cox, Ralph Scorza, and Karen Bussey

Armillaria root rot (ARR) of peach caused by the soil-borne basidiomycete fungus Armillaria tabescens is causing premature decline and mortality of peach trees on most southeastern U.S. peach farms. Soil inoculum may be present both in former peach orchard sites and on sites that were once in hardwood forest. The fungus is protected under the bark of dead root pieces and may survive up to 100 years at various depths in the soil profile. No commercially available rootstocks are resistant to ARR. Since 2002, we have embarked on a multipronged strategy to develop control options to combat ARR. First, we have two replicated trials on commercial grower replant sites with a history of ARR. Trial 1 compares four preplant fumigation treatments (none, Telone II, methyl bromide, and Enzone), three rootstocks (Lovell, Halford, and Guardian) and preplant root dips with endomycorrhizal fungi. Trial 2 compares the use of raised beds, root collar excavation and preplant root dips. Both trials examine long-term productivity and tree survival. Second, we are examining the use of systemic fungicide injection into infected trees to protect trees around infection foci. Third, we are trying to develop a genetically modified ARR-resistant rootstock. We have inserted the gene encoding the gastrodia antifungal protein (GAFP—a low molecular weight lectin that binds mannose and chitin) from a Chinese orchid into tobacco (model herbaceous system) and plum (model Prunus system). GAFP has antifungal activity against several basidiomycete root rot pathogens. Pathogenicity tests with transformed tobacco plants show enhanced tolerance to several root rot pathogens when compared to nontransformed plants. Transformed plums are being multiplied for pathogenicity tests.

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Kristin A. Schneider and James D. Kelly

Root rot, caused by Fusarium solani f.sp. phaseoli, is a serious disease of bean for which successful control has been elusive. Genetic resistance to the pathogen is considered quantitative and is strongly influenced by environmental factors. To reduce environmental variation and facilitate selection in earlier generations, an accurate, consistent, and nondestructive greenhouse screen was developed for the evaluation of Fusarium root rot resistance in bean. We describe a protocol that involves the germination of seedlings in perlite, inoculation of roots and hypocotyls 10 days after planting and evaluation within 4 weeks. The accuracy of this greenhouse screen was confirmed by demonstrating significant correlations between greenhouse and field ratings. Two experiments that included 24 and 21 diverse bean genotypes, respectively, were performed in the greenhouse and the ratings were correlated with field ratings over two growing seasons. Correlation coefficients between the greenhouse and field ratings were significant and as high as 0.99. Numerous genotypes can be evaluated within a short time for relatively minimal costs and labor. Furthermore, once roots have been rated and dipped in fungicide, plants can be transplanted for production of seed. This simple, rapid, and inexpensive protocol reduces environmental variation inherent to field ratings, thereby more accurately representing physiological resistance while maintaining a close association with observed field ratings.