Pecan scab (Seyran et al., 2010), caused by Fusicladium effusum G. Winter, is the most important disease of pecan cultivated in humid environments (Wood and Reilly, 1999). Almost all cultivated genotypes exhibit a degree of scab susceptibility under conditions favoring infection (Goff et al., 2003). With more susceptible cultivars, wet conditions can result in severe epidemics (Sparks et al., 2009). Other environmental factors (e.g., soil moisture and temperature) affect timely availability of nutrient elements, which may also affect susceptibility to pecan scab, as occurs in other crops with either visual or physiological nutrient deficiencies (Huber and Graham, 1999).
The susceptibility of pecan leaves to infection by F. effusum is greatest when foliage is young (≈18–28 d old or less) (Gottwald, 1985; Turechek and Stevenson, 1998; Wood et al., 1988). Scabbed foliage, shoots, and fruit can exhibit lower photoassimilation (Gottwald and Wood, 1985), yet it is the physical damage to developing fruit that makes the disease especially problematic. Infection can result in fruit abortion, poor kernel filling, smaller nuts/kernels, and altered nutmeat composition. Scab control in commercial orchards typically requires 3–18 fungicide cover sprays (Gottwald, 1985; Sparks, 1996; Turechek and Stevenson, 1998). Although appropriate fungicide use typically provides satisfactory scab control, protection is expensive, and disease control is often disappointing. In addition, fungicides might reduce carbon photoassimilation (Gottwald and Wood, 1985; Wood et al., 1985), which potentially influences flowering and crop load (Wood, 1989, 1995, 2011; Wood et al., 2003; Worley, 1979a, 1979b). Thus, there is need for improved scab disease management tools that increase efficacy and/or reduce control costs without adversely affecting tree health and production potential.
Toxicity, deficiency, or imbalances in either essential or beneficial nutrient elements can theoretically influence host susceptibility to fungal diseases through disruption of metabolic or physiological processes conferring disease resistance (Graham, 1983; Huber and Graham, 1999). Because timely availability of nutrient elements can influence disease severity, ensuring optimal nutritional physiology of cells, tissues, and organs may reduce scab incidence and severity. Nickel is an essential nutrient element often disregarded by nutrient management programs, although it is integral to certain essential metabolic processes (Bai et al., 2006, 2007, 2008). Pecan appears to possess a relatively high Ni requirement with factors such as soil environment, weather, and certain orchard management factors potentially triggering transitory early-season Ni deficiency in orchard trees (Nyczepir et al., 2006; Wood, 2010; Wood et al., 2004a, 2004b, 2004c, 2006) when tissues of foliage, shoots, and fruit are most susceptible to scab infection.
As a transition metal physiochemically similar to copper (Cu)—an effective scab fungicide (Demaree and Cole, 1927)—Ni might also possess direct toxicity to F. effusum. Indeed, the fungicidal efficacies of Ni compounds were apparent by 1908, and by 1963, there were 149 or more scientific references noting Ni activity against certain fungal species (Anonymous, 1964). Nickel salts are especially efficacious with a U.S. patent (No. 2,971,880) issued to Rohm and Haas Co. (Keil and Frohlich, 1961) for use of Ni as a fungicide. Thus, timely foliar Ni application during canopy expansion for improving tree nutritional physiology, a growth phase when susceptible hosts are most likely to be infected, might confer benefits indirectly by increasing host resistance and directly by fungicidal activity against F. effusum. This study assesses efficacy of foliar Ni application in pecan orchards for managing pecan scab and its potential as an integrated pest management tool.
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