Cultural controls for suppressing Phytophthora cinnamomi root rot of blueberry Public Deposited

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  • Phytophthora cinnamomi is a soilborne pathogen that causes root rot disease of highbush blueberry (Vaccinium corymbosum L.). When new installations of susceptible blueberry cultivars are infected with P. cinnamomi, plants often fail to grow significant new tissue, greatly reducing yields over the life of the planting. Chemical fungicides are available for disease suppression including mefenoxam and phosphonate compounds. However these tools are not available for organically certified growers. Initially, this research focused on non-chemical control strategies for blueberry root rot disease, including cultivar selection, soil amendments (gypsum and a variety of organic materials) via a series of trials in the greenhouse. Based on the findings of the greenhouse experiments, a field trial was conducted to evaluate a combination of cultural factors in a two year field experiment. Pre-plant gypsum incorporation into soil was evaluated as a control strategy in the field experiment in combination with other cultural practices that were hypothesized to affect disease including mulch type (geotextile weedmat or sawdust), and drip irrigation line placement. Eighteen highbush blueberry cultivars and advanced breeding selections were evaluated for susceptibility or resistance in three greenhouse experiments. Cultivars varied widely in susceptibility, with 'Duke,' 'Draper,' 'Bluetta,' 'Blue Ribbon,' 'Cargo,' 'Last Call,' 'Top Shelf,' and 'Ventura' exhibiting high levels of susceptibility to the disease. More resistant cultivars included 'Legacy,' 'Liberty,' 'Aurora,' 'Overtime,' 'Reka,' and 'Clockwork'. By selecting cultivars with superior resistance, growers may avoid yield losses associated with the disease, enhancing production profitability. Although choosing a disease resistant cultivar is likely the most effective control strategy for blueberry, other cultural disease control practices are needed when susceptible cultivars are grown to fill market demands. Fungicides can control disease in conventional plantings, but cannot be applied within certified organic production systems. Gypsum and organic amendments sometimes provide suppression of Phytophthora root rot in crops other than blueberry. Three greenhouse trials were conducted to evaluate organic soil amendments (peat, sawdust, dairy solids compost, yard debris compost, and composted municipal biosolids blended with douglas-fir bark) incorporated at 20% v/v into soil, and gypsum (CaSO₄) incorporated at 5% (v/v) for disease suppression in factorial combination. Trials were conducted with a disease susceptible cultivar ('Draper'), and soil moisture was maintained near saturation, favoring disease development. Organic amendments did not suppress disease in any of the experiments. Gypsum was effective in disease suppression in one of the three experiments. Gypsum provides a source of soluble calcium, and previous reported studies have demonstrated a mechanism for calcium-mediated suppression of P. cinnamomi. An additional greenhouse study was conducted to determine the relationship between gypsum application rate, soluble calcium in soil solution, and disease suppression. Soluble Ca in soil solution reached a plateau concentration of ~454 mg Ca L⁻¹ soil solution at gypsum application rates equal to or above 16 meq gypsum 100g⁻¹ soil. Higher gypsum application rates did not increase soil solution Ca, or provide additional disease suppression. Rates of gypsum required for disease suppression increase soil electrical conductivity (EC) beyond current recommended salinity guidelines for blueberries (> 2.0 mS/cm). The effects of salinity on plant growth were evaluated in a six month greenhouse experiment, using gypsum (CaSO₄) or potassium sulfate (K₂SO₄) salts to increase EC. Treatment EC levels ranged from 0.3 to 2.6 mS/cm, in ten increments. The maximum EC value chosen for this experiment approximates the maximum value for gypsum solubility in soil solution. Aboveground plant biomass declined more rapidly when EC was supplied by potassium sulfate than it did with gypsum. Root biomass declined when EC was supplied by potassium sulfate, but it was not affected by gypsum rate. Leaf cation concentrations responded strongly to increasing rates of potassium sulfate application. Leaf K increased dramatically, accompanied by declines in leaf Ca and Mg concentration. In contrast, leaf cation concentrations were much more stable when EC was adjusted by gypsum addition. Under the conditions of this greenhouse experiment, the increase in EC accompanying gypsum application had only minor effects on plant growth and nutrient uptake, suggesting that gypsum application is a viable option for trial in the field. A two year field trial was conducted to evaluate cultural practices for efficacy in suppressing P. cinnamomi root rot disease. Pre-plant gypsum incorporation into soil was evaluated as a control strategy in combination with other cultural practices that were hypothesized to affect disease: mulch type (geotextile weed mat vs. sawdust), and drip irrigation line placement. Disease suppression over two growing seasons was evaluated with the highly susceptible cultivar 'Draper' grown on a site with clay loam soil. P. cinnamomi inoculum was mixed into soil before planting. Experimental design was a 2x2x2 factorial with 2 mulch types (geotextile weed mat or douglas-fir sawdust), 2 drip line placements (narrow or wide placement relative to plant row), and 2 gypsum rates (0 and + gypsum). Gypsum was incorporated into planting beds in a 30-cm band at an application rate of 22,420 kg ha⁻¹ in-band equivalent to 2,242 kg ha⁻¹ on a whole-field basis. Drip irrigation treatments consisted of two drip lines placed either adjacent to the plant crown or 20-cm on either side of the crown (wide placement). A fungicide treatment was included to provide an assessment of the efficacy of cultural disease control vs. a chemical alternative. The fungicide treatment was mulched with sawdust and irrigated with drip lines adjacent to plants, and did not receive gypsum application. Mulch type had no significant effect on plant biomass after two years, but plants grown under sawdust had slightly higher biomass and less root infection. Soluble Ca, as measured by mini-lysimeters placed in the rootzone, was increased by gypsum application, especially with wide placement of drip irrigation lines. The disease suppressive effect of gypsum amendment depended on irrigation line placement. Soluble Ca movement in soil was associated with water movement away from drip emitters. When drip irrigation lines were placed adjacent to the plant crown, soluble Ca was moved away from the plant with the wetting front. With wide drip line placement , soluble Ca was moved toward the roots. Plants grown with wide drip line placement and gypsum addition had the lowest root infection incidence and highest plant biomass, likely as the result of more soluble Ca in the rootzone. Despite significant increases in plant biomass with gypsum and widely-placed irrigation lines, plants treated with conventional fungicide had approximately twice as much biomass after two growing seasons. An integrated control program is required for cultural suppression of blueberry root rot disease. Organic production using highly susceptible cultivars in the presence of P. cinnamomi is difficult, and may not produce equivalent yields to conventional production. Improved plant performance in the presence of P. cinnamomi was observed in these trials using cultivar resistance, gypsum, and widely-spaced drip irrigation lines. Other cultural practices, such as careful irrigation scheduling, and appropriate rate and timing of N fertilizer application are also important for disease suppression. In the future, greater P. cinnamomi disease suppression should be possible by using the disease suppressive cultural practices identified in this research in combination with a disease-resistant cultivar.
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