Fighting Phytophthora In Blueberries

The effects of Phytophthora root rot on blueberry plants are clearly evident. (Photo credit: Jerry Weiland, USDA-ARS.)

The effects of Phytophthora root rot on blueberry plants are clearly evident. (Photo credit: Jerry Weiland, USDA-ARS.)

The soilborne pathogen Phytophthora cinnamomi is associated with root rot in many woody perennial plants, but in an extensive study, we have identified a number of highbush blueberry cultivars that show resistance to the disease, as well as those varieties that are susceptible.

Phytophthora cinnamomi is a ubiquitous pathogen in blueberry production. Symptoms of infection can include poor shoot growth, root necrosis and dieback, yellowing or reddening leaves, and, in the most severe cases, dead canes and branches, and plant death. Phytophthora cinnamomi infection is more prevalent in plants treated with drip irrigation rather than sprinklers, and is most common in heavy soils and poorly drained sites.

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We conducted two experiments: one with 10 commercially established highbush blueberry cultivars, and the other with seven newly released cultivars, three commercially established cultivars, and three advanced selections.

Four commercially established cultivars (‘Aurora’, ‘Legacy’, ‘Liberty’, and ‘Reka’), and two new cultivars (‘Overtime’ and ‘Clockwork’) were identified as the most resistant to Phytophthora cinnamomi.

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For the first experiment, 12 plants of each cultivar were obtained from a nursery as 5-centimeter-diameter tissue culture plugs (Fall Creek Nursery, Lowell, OR), transplanted into 2.6-liter containers filled with potting medium (Metro Mix 840PC; Sun Gro Horticulture, Agawam, MA), inoculated with the pathogen, and placed on two greenhouse benches in September 2011.

All cultivars were one-year-old and 10 to 15 centimeters tall at transplanting. The greenhouse was maintained at a constant 20°C day/night air temperature and equipped with 1,000-watt, high-pressure, sodium lamps. The experiment was harvested 15 weeks after transplanting.

The experiment was repeated in April 2012. The only difference during the second trial was that the greenhouse was maintained at a temperature of 25°C/20°C (day/night) with no supplemental lighting, and silver-colored, reflective shade-cloth (60% shade reduction) was placed over the top of the greenhouse to reduce the heat load. The inoculum also contained slightly more propagules and the plants were harvested at 20 weeks after transplanting.

The effects of Phytophthora root rot on blueberry plants are clearly evident. (Photo credit: Jerry Weiland, USDA-ARS.)

The effects of Phytophthora root rot on blueberry plants are clearly evident. (Photo credit: Jerry Weiland, USDA-ARS.)

The second experiment, evaluating newly released cultivars, advanced selections, and three commercially established cultivars (‘Draper’ and ‘Legacy’ as susceptible and resistant standards, respectively, and ‘Reka’) was conducted in September 2013 under similar conditions as described in our first experiment.

As expected, some cultivars were faster growers than others, resulting in greater accumulation of shoot and root biomass. However, the root and shoot biomass of healthy, noninoculated cultivars was not positively correlated with disease resistance, indicating that the more vigorous cultivars (i.e., those with greater biomass) were not necessarily those that were more resistant to the disease. For example, ‘Aurora’ was one of the slower cultivars, but was rated resistant, whereas ‘Bluetta’ was one of the fastest cultivars, but was rated susceptible.

The most resistant genotypes included four commercially established cultivars, ‘Aurora’, ‘Legacy,’ ‘Liberty,’ and ‘Reka,’ and two new cultivars, ‘Overtime’ and ‘Clockwork.’
”When these genotypes were inoculated (with Phytophthora cinnamomi), average relative shoot biomass was equal to or greater than 60% of that of the noninoculated plants, whereas relative root biomass was equal to or greater than 40%,” we concluded.

It is important to note that while these cultivars were more resistant to Phytophthora cinnamomi, they are not completely immune to the pathogen: some loss in root and shoot biomass was still observed compared to noninoculated, healthy plants, but the loss was not as great as that in the susceptible cultivars.

Results showed that relative shoot biomass of the susceptible genotypes, on the other hand, ranged from 19% to 53%, while 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.’

Genetic resistance can be helpful for minimizing economic losses on sites where Phytophthora root rot has been a problem. Growers should avoid planting susceptible cultivars on sites where the soil conditions are conducive to root rot (e.g., heavy soils and/or poor drainage). Resistant cultivars can help minimize disease on these sites.

However, even resistant cultivars may develop root rot if there is high inoculum pressure and conditions are conducive. Young plants are considered the most susceptible, so fungicides may be needed initially during the establishment phase to help resistant plants reach productive maturity.

Harvesting Times

Here are the cultivars in the study, all highbush, in order of ripening:

■ ‘Ventura’ (very early-season southern)
■ ‘Bluetta’ (early-season northern)
■ ‘Duke’ (early-season northern)
■ ‘Reka’ (early-season northern)
■ ‘Star’ (early-season southern)
■ ‘Blue Ribbon’ (early-to-mid-season northern)
■ ‘Draper’ (midseason northern)
■ ‘Bluegold’ (midseason northern)
■ ‘Clockwork’ (mid-season northern)
■ ‘Top Shelf’ (mid-season northern)
■ ‘Bluecrop’ (midseason northern)
■ ‘Legacy’ (mid-to-late-season northern)
■ Liberty’ (late-season northern)
■ ‘Cargo’ (late-season northern
■ ‘Elliott’ (very late-season northern)
■ ‘Aurora’ (very late-season northern)
■ ‘Last Call’ (very late-season northern)

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