Soybean (Glycine max (L.) Merrill) is a host of Macrophomina phaseolina (Tassi) Goid., the necrotrophic ascomycete fungus that causes damping-off, seedling blight, collar, stem and root rot, and charcoal rot. In addition to soybean, M. phaseolina causes economic losses on sesame (Sesamum indicum L.), safflower (Carthamus tinctorius L.), sunflower (Helianthus annuus L.), and more than 500 cultivated and wild plant species in 75 families (Twizeyimana et al., 2012, Gupta et al., 2012). In the United States, charcoal rot is the third most damaging soybean disease in terms of yield loss after soybean cyst nematode and seedling diseases. Average yield losses to this disease totaled 219.6 million bushels from 2010 to 2014 across 28 soybean-producing states (Allen et al., 2017). In Kansas, yield losses averaged 4.3% from 2010 to 2018, which ranged from an estimated 150,000 bu lost in 2015 (0.1%) to 15.5 million bu lost in 2012 (15.0%)
Resistance or tolerance in host plants is the best management strategy to reduce infection and damage from charcoal rot, although only a few reliable resistance sources have been found. Genetic resistance to SDS, Fusarium diseases, and charcoal rot occurs using different mechanisms, but varieties exist that exhibit moderate resistance or tolerance to these diseases. Ultimately, a catalog of varietal resistance will be required to integrate these (largely) quantitative traits into materials that can be released from the Kansas breeding program.
Otherwise, well-studied cultural management strategies that have been employed include controlling plant populations and weeds to reduce stress (Bowen and Schapaugh, 1989), crop rotation to non-host cereals to reduce soil M. phaseolina populations (Mueller et al., 1985), balancing soil fertility and phosphorus management to improve yield under pathogen pressure (Gupta et al., 2012), using no-till as a way to conserve moisture and reduce soil temperatures (Mengistu et al., 2009), and irrigation management in systems where relevant or practical (Cruz, 2011; Gupta et al., 2012).
Our research has demonstrated the of high-glucosinolate mustard as a cover crop reduces M. phaseolina populations in soybean plants and soil (Sassenrath et al., 2017). However, no such studies have been done to test the effect of this cover crop plant for control of SDS or other Fusarium spp. that cause root and seedling diseases. Many plants produce biofumigant chemicals that control or reduce harmful soil fungi, including M. phaseolina. For example, Mengistu et al. (2009) showed some suppression of charcoal rot infestation with reduced tillage and use of rye (Secale cereale L.) as a cover crop. Plants in the Brassicaceae family produce a type of biofumigant that has been used to control fungal soil-borne pathogens (Matthiessen and Kirkegaard, 2006). Some Brassica cover crops such as Brassica juncea L. release aliphatic and aromatic secondary metabolites, including various glucosinolates, whose breakdown products are potentially inhibitory to plant pathogens, promote shifts in the soil microbial community, and enhance nutrient cycling (Bones and Rossiter, 1996; Matthiessen and Kirkegaard, 2006; Weerakoon et al., 2012). Glucosinolates are sulfur-rich compounds that are broken down or transformed in the soil by the resident microbial community into isothiocyanates (ITCs). These compounds exhibit broad toxicity against many organisms due to their sulfur-rich content and high reactivity (Bones and Rossiter, 1996; Rask et al., 2000).
Fluopyram is the active ingredient in ILeVo brand seed treatments and has been shown to be highly effective against SDS disease and nematodes in the rhizosphere of soybean. Fusarium virguiliforme has been shown in direct to tests to be sensitive to this compound, although some fungicide tolerance existed in the pathogen population tested (Wang et al., 2017). Fortunately, Kansas F. virguiliforme isolates showed some of the lowest tolerance to fluopyram. Some research has been conducted to test the efficacy of this anti-fungal against various fungi in other crops. However, few studies have examined the effect of this a.i. against other Fusarium spp. that attack soybean seedling or roots, or the effect against charcoal rot.
The 2020 field season saw a notable spike in Phytophthora root rot incidence in southeastern soybean production areas. Over the years, this pathogen has appeared occasionally in the eastern part of the state with the greatest impacts observed in the north-eastern counties. However, the appearance of Phytophthora sojae in the warmer SE counties signals a potential pathogen shift or a change in the genetics of the varieties in the maturity groups used in the southern part of the state.