Phytophthora cinnamomi Rands is a soilborne pathogen that causes root rot in >900 plant species worldwide, including northern highbush blueberry (Zentmyer, 1980). Currently, there are two groups of fungicides (phenylamides and phosphonates) registered for root rot control in blueberry, including mefenoxam and fosetyl-Al (Brannen et al., 2009). Unfortunately, the risk of developing fungicide resistance is very high in Phytophthora species, especially to mefenoxam (Hu et al., 2010). In addition, organic blueberry production is increasing rapidly, but neither of these chemicals is certified for use on organic crops (DeVetter et al., 2015). Preplant soil fumigation is also an option for managing root rot. However, it is expensive, not widely used in the industry, and there is little evidence for its effectiveness against this disease in blueberry production systems. The risk of crop loss from P. cinnamomi may increase substantially without fungicides; therefore, disease-suppressive cultural practices are needed for both conventional and organic blueberry systems.
Organic mulches (e.g., sawdust or bark) have been shown to suppress soil populations of P. cinnamomi in eucalyptus (Eucalyptus sp. L'Hér.), fraser fir [Abies fraseri (Pursh) Poir.], and avocado (Persea americana Mill.) plantings (Downer et al., 2001; Menge et al., 1994; Richter et al., 2011a). The cell walls of Phytophthora hyphae are largely composed of glucans, and glucanase activity associated with the microbes in organic mulches may cause hyphal lysis and sporangial abortion (Downer et al., 2001; Richter et al., 2011b). Historically, most blueberry fields were mulched with sawdust or bark chips (Strik and Yarborough, 2005). However, many new fields are mulched with geotextile fabrics, often referred to as “weed mat.” Weed mat is more economical for weed control than sawdust, particularly in organic systems, and the use of it may result in greater plant growth and yield in blueberry (Julian et al., 2012; Larco et al., 2013). Most growers use black weed mat, which increases soil temperature during sunny days, especially when there is little shade under the canopy such as early in the season and in new plantings (Larco, 2010). Root infection by P. cinnamomi has been shown to be more severe when soil temperature exceeds 25 °C (Erwin and Ribeiro, 1996). Therefore, it is possible that weed mat may increase the incidence of phytophthora root rot in blueberry.
The zoospore infection cycle of P. cinnamomi requires nearly saturated soil moisture conditions (de Silva et al., 1999). Consequently, infection is readily affected by heavy rain or irrigation and is often greater when plants are irrigated by drip irrigation than by sprinklers or microsprinklers, which results in increased soil water content near the plant crown (Bryla and Linderman, 2007). Whereas most blueberry fields were historically irrigated by sprinklers, many are now irrigated by drip because of concerns for food safety and efforts to increase water use efficiency (Bryla et al., 2011). Many blueberry growers use two lines of drip tubing per row, with one line placed on either side of the plant (Ehret et al., 2012). The drip lines are usually placed near the plant crown during the first year or two after planting and are later moved 15–20 cm from the crown, once the root system develops (Bryla and Strik, 2015). However, given the propensity for developing root rot in wetter soils, Bryla and Linderman (2007) suggested that blueberry plants might benefit from a wider drip line spacing immediately after planting, particularly if the soil is already infested with P. cinnamomi.
Soil amendment with calcium may also suppress phytophthora root rot (Menge et al., 1994; Messenger et al., 2000a; Pinkerton et al., 2009). A high level of calcium in the soil inhibits zoospore motility of the pathogen and thereby disrupts the infection process (Messenger et al., 2000b). Greenhouse assays suggested that gypsum may be useful for reducing phytophthora root rot in northern highbush blueberry (Yeo, 2014). Gypsum provides more soluble Ca2+ than carbonate sources such as lime and does not increase soil pH appreciably (Shainberg et al., 1989). High soil pH is deleterious to many ericaceous plant species, including northern highbush blueberry, which is adapted to soil pH in the range of 4.5–5.5 (Hart et al., 2006).
The objective of the present study was to evaluate the effects of mulch type, drip line placement, and gypsum on phytophthora root rot in northern highbush blueberry. The study was carried out in a new planting, as young plants are usually the most susceptible to root rot (Bryla and Linderman, 2007).
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