A hydroponic method was developed and tested for screening red raspberry genotypes for resistance to Phytophthora fragariae var. rubi, the most common causal agent of Phytophthora root rot in raspberry. Plants of `Titan' and `Encore' exhibited typical disease symptoms, with the latter developing significantly smaller stem lesions and fewer petiole lesions. The resistant cultivar, `Latham', regenerated healthy root tissue from the crown and older-order roots after initial infection and necrosis of young roots and exhibited no other symptoms beyond minor leaf chlorosis. This component of the resistance reaction has not been documented previously. A segregating F1 population from the cross of `Latham' × `Titan' had a survival rate of 56% with 42% classified as resistant, exhibited minimal symptoms, and produced varying amounts of healthy root tissue. This screening method allows multiple observations of all plant tissues, including roots, under repeatable and definable growth chamber conditions. It should be useful for classifying the phenotype of individuals in segregating red raspberry populations to investigate the inheritance of Phytophthora root rot resistance using molecular markers.
J.A. Pattison, W.F. Wilcox, and C.A. Weber
Wendy K. Hoashi-Erhardt, Patrick P. Moore, Gwenyth E. Windom, and Peter R. Bristow
Red raspberry genotypes (Rubus idaeus L.) were evaluated for resistance to root rot at two field sites in Washington state and in a greenhouse study. Thirteen raspberry genotypes were planted in two field sites naturally infested with Phytophthora fragariae var. rubi Wilcox and Duncan and evaluated over 3 years for growth and symptom expression. In greenhouse pot tests, 14 genotypes were inoculated with an isolate of P. fragariae var. rubi at three inoculum levels and evaluated for growth, root color, and symptom expression using a 1 to 4 rating scale. Eleven of the 14 cultivars were found to be susceptible or very susceptible to root rot in the field and greenhouse. ‘Summit’ and ‘Newburgh’ possessed high levels of resistance to the pathogen. ‘Cascade Bounty’ also showed high resistance to root rot in the greenhouse, but confirmation from a field study is needed. Subjective root ratings of greenhouse-grown plants correlated well with measurements of cane numbers and cane infection in the field. The greenhouse tests were useful in identifying resistant genotypes and very susceptible genotypes but did not always match field results. Observation of at least 3 years in the field was necessary to compare relative reaction with root rot among genotypes.
B.K. Duffy and G. Défago
We thank Barbara Pärli and Ingeborg Kump for technical assistance, Jacques Fuchs and Marcello Zala for valuable advice in developing the disease assay, and Emmannuel Frossard, ETH Zürich, and David Weller, USDAARS Pullman, Wash., for
W. Patrick Wechter, Melanie M. McMillan, Mark W. Farnham, and Amnon Levi
individuals were selected from a greenhouse-based disease assay using a South Carolina Fon -R2 isolate. In our original greenhouse experiments, tolerance to Fon -R2 was significantly higher in these two PIs than in the watermelon cultivars Charleston Gray
Alexis K. Nagel, Guido Schnabel, Cesar Petri, and Ralph Scorza
disease assays and served as the negative control groups. Inoculated and uninoculated experimental plants were arranged in randomized complete blocks on benches in an air-conditioned, biosafety level 2 greenhouse (27 ± 5 °C). Plants were watered
Jareerat Chunthawodtiporn, Theresa Hill, Kevin Stoffel, and Allen Van Deynze
resistant sources, pepper populations, and disease assays have been used ( Bonnet et al., 2007 ; Kim et al., 2002 , 2008 ; Mallard et al., 2013 ; Minamiyama et al., 2007 ; Ogundiwin et al., 2005 ; Quirin et al., 2005 ; Rehrig et al., 2014 ; Sy et al
Kyle E. LaPlant, Gregory Vogel, Ella Reeves, Christine D. Smart, and Michael Mazourek
challenges for cucurbit growers. As a result, breeding for resistance to phytophthora crown and root rot in squash and pumpkin began in 2011 at Cornell University (Ithaca, NY). Using disease assays conducted on seedlings in the greenhouse, we performed