Pumpkin is a globally important cash crop grown for the processing and fresh-market industries (Ingerson-Mahar et al., 2007). Nearly 2 billion pounds of pumpkin were harvested in the United States in 2017 (Gregory, 2018). One of the major problems associated with pumpkin is the risk of premature defoliation caused by foliar diseases such as powdery mildew. Podosphaera xanthii (formerly Sphaerotheca fuliginea) and Erysiphe cichoracearum are the two reported fungal species that can cause powdery mildew in cucurbit crops in the United States (Zitter et al., 1996). These pathogens can move long distances within the growing season, from southern to northern U.S. production areas (Zitter et al., 1996). Cucurbit powdery mildew infects leaves and vines at any growth stage, typically starting with the older leaves. Symptoms of powdery mildew include white colonies to large, coalesced white blotches on leaves causing chlorosis, and is eventually followed by loss of foliage. Powdery mildew can significantly reduce the yield of pumpkins both in terms of fruit size and number (Mossler and Nesheim, 2014; Zitter et al., 1996). Conventional and organic cucurbit growers take substantial efforts to control or mitigate losses to powdery mildew. Weekly applications of a fungicide can result in significant increases in cost, equipment, time, and labor. Most conventional fungicides currently used for cucurbit powdery mildew control have a high risk for resistance development (Wyenandt et al., 2018). The risk of losing fungicide efficacy for controlling diseases such as cucurbit powdery mildew requires continued efforts to help mitigate disease development through alternative means. Organic growers have fewer effective control options and face greater challenges when dealing with this pervasive disease. Organic growers can grow resistant or tolerant pumpkin cultivars, but additional disease control options are needed.
An approach that has gained attention recently includes improved soil fertility management and optimized plant nutrition (Datnoff et al., 2007). In particular, the application of Si as part of a fertilization strategy has been studied for typical Si accumulator species such as rice (Oryza sativa), wheat (Triticum aestivum), and cucurbits (Belanger et al., 2003; Elawad and Green, 1979; Heckman et al., 2003; Lepolu et al., 2016; Provance-Bowley et al., 2010). A review by Datnoff (2014) summarized the current understanding of the physiological significance of Si in plants. Si increases plant resistance to fungal diseases by either increasing the Si content in epidermal tissue, thus forming a thickened Si–cellulose layer that is more resistant to fungal penetration, or by pathogenesis-mediated host defense responses (Zellner, 2017). In addition, a variety of crops, especially Si accumulators, showed increases in biomass, Si accumulation, and disease or pest resistance when treated with plant-available Si (Zellner et al., 2011, 2019). Although not officially regarded as an essential plant nutrient, Si is now widely considered a beneficial element for many plants (Datnoff, 2014; Datnoff et al., 2001). Several plant growth media companies have started to incorporate Si in their soil-less products.
Both conventional and organic growers are interested in the types and application rates of approved Si materials that can adequately address disease problems. Acquiring naturally derived and approved organic sources of Si for organic production has become a priority. In previous studies, members of our group identified and investigated the properties of several Si mineral sources, including earth-mined minerals such as wollastonite, MontanaGrow (MontanaGrow, Bonner, MT); glacial rock flour; and human-processed minerals such as wood ash and steel mill slag (Heckman et al., 2003; Lepolu et al., 2016). We used pumpkin as a model crop and investigated the beneficial effects of different amounts of Si amendments, including each amendment’s ability to neutralize soil acidity, enhance Si uptake, improve powdery mildew control, and increase plant biomass. Wollastonite, a naturally occurring mineral form of calcium silicate (Ca2SiO4), can provide all these tested beneficial effects to pumpkin plants. This product is naturally mined, and is listed by the Organic Materials Review Institute (OMRI; Eugene, OR) for use in organic production systems. We conducted experiments to understand further the effects of wollastonite on soil and plants under disease conditions, and to provide useful information to growers and the plant growth media industry. The objectives of our study were 1) to find the optimal soil amendment rate for wollastonite to achieve the best suppression of powdery mildew, 2) to determine wollastonite’s ability to neutralize soil acidity and change soil chemistry compared with regular limestone, and 3) to investigate the biomass accumulation in pumpkin plants resulting from wollastonite soil applications.
Rates for liming material application are often determined based on initial soil pH, target soil pH for the crop, and the liming requirement to reach that target. However, agronomists specializing in soil fertility not only need to provide sound advice on making optimum application rates of soil amendments, but also need to predict potential impacts on plant growth and crop mineral nutrition when target application rates are exceeded. Therefore, our greenhouse study was designed to include a wide range of wollastonite application rates, ranging from an unamended soil in need of liming, to a level that matched the lime requirement of the soil for growing pumpkin and most vegetable crops, as well as levels several orders of magnitude greater. Another reason for exploring greater application rates is that pumpkins are typically grown in widely spaced rows, permitting localized heavier application rates in the areas of seeding or transplanting that then are later dispersed by tillage. Application rates of wollastonite that might at first appear extremely high are more reasonable when one considers that future tillage can disperse the amendment across the field and extend the benefit to successive crops.
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