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Root rot caused by the soil-borne pathogen Phytophthora cinnamomi is one of the deadliest and most costly diseases in rhododendron culture. Unfortunately, the majority of cultivars appear to be susceptible to this fungus. Host resistance does occur, but it represents a tolerance of rather than immunity from the disease. A breeding program has been initiated to develop a broader array of root rot resistant cultivars and to determine the genetic basis for resistance. Greenhouse inoculations and screenings of 48 contemporary cultivars yielded seven clones with moderate to high levels of resistance to P. cinnamomi. Protocols for evaluation at the seedling stage were developed in order to screen large breeding populations of about 200 seedlings per cross. Root rot tolerance appears to have low-moderate heritability in these rhododendron populations. Groups of progeny with one resistant parent had a slower mortality rate and higher survivorship (avg. 10%) after 2 months of disease pressure than crosses in which both parents were susceptible (0 survivorship). A recurrent selection strategy is planned to increase the frequency of alleles for resistance in breeding populations of rhododendrons.
Seven highbush blueberry (Vaccinium corymbosum L.) cultivars were evaluated for their photosynthetic heat stability. Ail showed significant reductions in CO2 assimilation rates (A) as leaf temperatures were raised from 20 to 30C, although `Blue-crop', `Jersey', `Elliot', and `Rubel' (22% to - 27%) were significantly less affected than Spartan', `Bluejay', and `Patriot' (-41% to -51%). To determine whether temperature adaptations of highbush types can be broadened through hybridization with native, heat-tolerant species, `Bluecrop' was crossed with the V. darrowi Camp. selection Florida 4B, and F2, BC1, and BC2, populations were generated. This approach showed promise as genotypes were identified in all the derivative populations that were more heat tolerant than `Bluecrop' and had a high A.
Abstract
Flower bud injury was assessed in 18 cultivars of highbush blueberry (Vaccinium corymbosum L.) after two spring frosts. Bud position on shoots was significantly correlated (P < 0.05) with percent brown ovaries. Significant differences in proportion of brown ovaries were noted among cultivars, but most of the variation was associated with stage of bud development. The least-developed buds were the most hardy. Bloom date was significantly correlated with harvest date across cultivars, although ‘Spartan’ flowered much later than other early ripening cultivars.
Flower bud and leaf samples collected from a wide range of native North American Vaccinium populations were tested for the presence of blueberry shoestring virus (BBSSV) using the enzyme-linked immunosorbant assay. The highest disease incidence was found in Michigan (14%), although a few positive samples also were found in Virginia, New Jersey, Maine, Ontario, and Quebec. Of seven species tested, only V. corymbosum L. and V. angustifolium Ait. were infected with BBSSV.