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John R. Yeo, Jerry E. Weiland, Dan M. Sullivan, and David R. Bryla

Phytophthora cinnamomi Rands causes root rot of northern highbush blueberry (Vaccinium corymbosum L.), which decreases plant growth, yield, and profitability for growers. Fungicides are available to suppress the disease, but are prone to development of resistance in target pathogens and cannot be used in certified organic production systems. Alternative, nonchemical, cultural management strategies were evaluated to reduce phytophthora root rot in a field infested with P. cinnamomi. The field was planted with ‘Draper’ blueberry, which is highly susceptible to the pathogen. The soil was either amended with gypsum or not before planting, and the plants were irrigated using narrow (adjacent to plant crown) or widely spaced (20 cm on either side of the plant crown) drip lines and mulched with douglas fir sawdust or black, woven geotextile fabric (weed mat). A fungicide control treatment was also included in the study and consisted of applying two conventional fungicides, mefenoxam and fosetyl-Al, to plants irrigated with narrow drip lines and mulched with sawdust. Initially, root infection by P. cinnamomi was lower with the combination of gypsum, wide drip lines, and sawdust mulch than with any other treatment, except the fungicide control. The soil under weed mat accumulated more heat units than under sawdust and resulted in faster hyphal growth by the pathogen. However, plant growth was similar in both mulch types. The effects of drip line placement and gypsum, on the other hand, were interactive, and plants grown with a combination of wide drip lines and gypsum produced the greatest amount of biomass among the cultural treatments. Narrow drip lines negated the disease-suppressive effects of gypsum by moving zoospore-inhibiting Ca2+ away from the plant root zone, and also resulted in wetter soil near the crown of the plants, which likely promoted zoospore discharge and root infection. However, wide drip lines resulted in N deficiency symptoms during the first year after planting and, therefore, resulted in less plant growth than the fungicide control. Thus, if N is managed properly, this study suggests that concerted use of gypsum and wide drip lines can help suppress phytophthora root rot in northern highbush blueberry and increase production in field soils where the pathogen is present.

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John R. Yeo, Jerry E. Weiland, Dan M. Sullivan, and David R. Bryla

Phytophthora cinnamomi Rands is a ubiquitous soilborne pathogen associated with root rot in many woody perennial plant species, including highbush blueberry (Vaccinium corymbosum). To identify genotypes with resistance to the pathogen, cultivars and advanced selections of highbush blueberry were grown in a greenhouse and either inoculated or not with propagules of P. cinnamomi. Two experiments were conducted, including one with 10 commercially established cultivars and another with seven newly released cultivars, three commercially established cultivars, and three advanced selections of highbush blueberry. Pathogen resistance was based on the shoot and root dry biomass of the inoculated plants relative to the noninoculated plants within each genotype, as well as on the percentage of root infection among the genotypes. Resistant genotypes included four commercially established cultivars, Aurora, Legacy, Liberty, and Reka, and two new cultivars, Overtime and Clockwork. When these genotypes were inoculated, average relative shoot biomass was ≥60% of that of the noninoculated plants, whereas relative root biomass was ≥40%. ‘Star’, as well as two advanced selections (an early- and a late-season type) may also have some degree of resistance, but further investigation is needed. Relative shoot biomass of the susceptible genotypes, on the other hand, ranged from 19% to 53% and relative root biomass ranged from 11% to 26%. The susceptible genotypes included ‘Bluetta’, ‘Bluecrop’, ‘Bluegold’, ‘Blue Ribbon’, ‘Cargo’, ‘Draper’, ‘Duke’, ‘Elliott’, ‘Last Call’, ‘Top Shelf’, and ‘Ventura’. These cultivars are not recommended at sites with conditions conducive to root rot, such as those with clay soils and/or poor drainage.