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Valérie Gravel, Claudine Ménard, and Martine Dorais

Mandelbaum, 1986 ; Lumsden et al., 1983 ; Zhang et al., 1996 ). Considering that geranium plants are susceptible to root diseases caused by a multitude of pathogens, including numerous Pythium species such as P. splendens ( Griffin, 1972 ), P. ultimum

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Greg T. Browne, Joseph A. Grant, Leigh S. Schmidt, Charles A. Leslie, and Gale H. McGranahan

highly susceptible to P. cinnamomi and P. citricola and developed high levels of root and crown rot (68% to 100%; Table 2 ) and significant reductions in root fresh weight in soil with the pathogens ( P = 0.05). In contrast, the WN selections were

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Bryan K. Sales, David R. Bryla, Kristin M. Trippe, Jerry E. Weiland, Carolyn F. Scagel, Bernadine C. Strik, and Dan M. Sullivan

pathogens, including P. cinnamomi ( Zwart and Kim, 2012 ), which is commonly associated with root rot in highbush blueberry ( Bryla et al., 2008 ; Yeo et al., 2016 ). Recent studies suggest that biochar should be combined with other sources of organic

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Susan C. Miyasaka, Charles E. McCulloch, and Scot C. Nelson

)]. Among these, nine originated from Palau and 24 were from Hawai‘i. Corm rots could be caused by many different oomycete and fungal pathogens, including Pythium sp., P. colocasiae , and Sclerotium rolfsii ( Ooka, 1994 ). In another study ( Miyasaka et

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Ariadna Monroy-Barbosa and Paul W. Bosland

blight, fruit rot, root rot, and stem blight ( Sy et al., 2005 ); and for the foliar blight, stem blight, and root rot disease syndromes, the resistance is independently inherited ( Sy et al., 2005 ; Walker and Bosland, 1999 ). In addition, several

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Hetal M. Kalariya, Guido Schnabel, Cesar Petri, and Ralph Scorza

years. Creating rootstocks for fruit trees with resistance to fungal root pathogens and nematodes is a desirable component of Integrated Pest Management practices. Classical breeding has in the past been the only method available to develop disease

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M. Taylor Perkins, Anna Claire Robinson, Martin L. Cipollini, and J. Hill Craddock

resistance to phytophthora root rot (PRR). ( B ) Crossing strategy used to generate better-backcross (BB 1 ) seedlings screened for resistance to PRR. Table 1. Hybrid Castanea families screened for phytophthora root rot resistance. Hybrid family names

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J.O. Kuti, G.V. Latigo, and J.O. Bradford

Soil-borne pathogens such as Macrophomina phaseolina (the causative agent of charcoal rot) and Phymatotrichum omnivorum (the causative agent of cotton root rot) contribute to mortality of transplanted guayule (Parthenium argentatum, Gray) seedlings in southern Texas. In order to select guayule genotypes for resistance to these pathogens, it would be useful to develop reliable greenhouse inoculation procedures for screening guayule seedlings. Twelve-week-old guayule seedlings (`11591', a USDA standard breeding line) were inoculated using two inoculation methods (soil-drenching and root-dipping) in two soil media (field soil and commercial soil mix). Plants were rated for disease severity 2 to 5 months after inoculation and pathogens were re-isolated from diseased plants to establish Koch postulates. The soil drenching technique, using field soil, caused rapid development of disease symptoms that were consistent with re-isolation frequencies of pathogens from the diseased plant tissues.

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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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K.A. Jacobs and G.R. Johnson

Seedlings of eight Prunus taxa were evaluated for variation in susceptibility to a single, 4- or 5-day flooding period and root rot caused by Phytophthora cryptogea Pethybr. & Lafferty. Survival, plant defoliation, disease severity index, root necrosis, and net photosynthesis indicated that the combination of flooding and pathogen was significantly more severe to all taxa than either individual treatment. Most response variables reflected early plant dysfunction but were not correlated with long-term survival. Long-term survival was 70% in the combination treatment compared to 99% in the control group. Flooding injured seedlings more than the pathogen in most taxa. Taxa differed only slightly in tolerance to the treatments, as measured by survival rate. Prunus takesimensis Nakai had the highest survival rate of 100% and along with P. mahaleb L. and P. yedoensis Matsum. showed some tolerance to flooding and the pathogen. Prunus sargentii Rehd. had the lowest survival rate of 81% and appeared to be least tolerant to the pathogen.