Assessing the Resistance of Red Raspberry (Rubus idaeus L.) Genotypes to Phytophthora fragariae var. rubi in Hydroponic Culture

Abstract

<jats:p>A hydroponic method was developed and tested for screening red raspberry genotypes for resistance to <jats:italic>Phytophthora fragariae</jats:italic> var. <jats:italic>rubi,</jats:italic> 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 F<jats:sub>1</jats:sub> 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.</jats:p>

Field and Greenhouse Response of Red Raspberry Genotypes to Root Rot

“…'Cascade Bounty' had resistance similar to that of 'Newburgh' in the greenhouse, which agrees with earlier field observations (Moore and Finn, 2007), although confirmation is needed from a replicated field study. Pattison et al (2004) observed that a cultivar's ability to generate new primocanes from root tissue rather than existing crown tissue may allow it to recover from moderate or intermittent root rot infection in the field. However, such a recovery mechanism may not be apparent in a short-term greenhouse test with high disease pressure, leading to a susceptibility rating higher than observed in the field (Pattison et al, 2004).…”

“…A few cultivars are resistant to root rot such as 'Newburgh' and 'Latham', but most commercial cultivars are susceptible to root rot and eventually show disease symptoms (Pattison et al, 2004). These include 'Meeker', which accounts for more than 70% of raspberry plantings in the Pacific Northwest (Washington Red Raspberry Commission, 2004), and 'Willamette' (Barritt et al, 1981).…”

“…However, field evaluations are time-consuming, as symptoms may take several years to develop, and soil conditions and disease pressure may not be uniform. Greenhouse pot tests (Bristow et al, 1988;Kennedy and Duncan, 1991;Laun and Zinkernagel, 1993;Wilcox et al, 1999a) and hydroponic culture systems (Pattison et al, 2004) can provide greater control of environment and disease pressure, but typically use small plants that may not respond in the same manner as mature, well-established plants under field conditions. 'Cowichan' was moderately resistant to root rot in a greenhouse pot test but was shown to be susceptible in subsequent field plantings (Kempler et al, 2005).…”

Raspberry

“…Above-ground symptoms of root rot usually appear within 10 weeks and susceptible genotypes are dead within 15 weeks while resistant seedlings grow vigorously (Daubeny 1996 ) . Pattison et al ( 2004 ) developed an effective hydroponic procedure to conduct screening for resistance. Young seedlings can be pre-fi eld screened for resistance to other diseases and pests according to the relative importance of the problem and the practicality of the screening.…”

“…The sources of resistance used in this breeding are from 'Cherokee' and 'Latham' and, by selecting in fi elds heavily infested with P. rubi , they have identifi ed individuals with high fi eld resistance. The New York program is screening for P. rubi in the fi eld and in hydroponic systems in the greenhouse (Pattison et al 2004 ) . 'Prelude,' 'Heritage,' and 'Taylor' show good fi eld resistance to P. rubi .…”