Isolation and Identification of Fusarium solani f. sp. glycines from Soil on Modified Nash and Snyder's Medium

Abstract

Modified Nash and Snyder's medium (MNSM) has been used to study soil populations of the fungus Fusarium solani f. sp. glycines, the causal agent of sudden death syndrome (SDS) of soybean. However, no studies have been conducted to confirm the accuracy of this technique in enumerating the SDS pathogen. To determine what portion of the colonies enumerated on MNSM are the SDS pathogen, 282 isolates resembling F. solani f. sp. glycines were collected from soil of two fields with a history of SDS in Arkansas using a soil dilution method with MNSM. The colony morphology of these isolates was compared on MNSM and on potato dextrose agar (PDA). Of these isolates, 112 were tested for pathogenicity on soybean in the greenhouse. In addition to fungi that produced colonies typical of F. solani f. sp. glycines on MNSM, 5 isolates each of four common colony types on MNSM were collected from each field for a total of 40 isolates. These isolates were compared to F. solani f. sp. glycines for colony morphology on PDA and for pathogenicity. Isolates that had colonies on MNSM that resembled F. solani f. sp. glycines had colony morphologies on PDA similar to this pathogen and produced SDS-like foliar symptoms in greenhouse inoculations of soybean. There was a significant quadratic relationship between foliar symptom severity and plant fresh weight. None of the other isolates tested resembled F. solani f. sp. glycines on either MNSM or PDA or produced SDS-foliar symptoms in greenhouse inoculations. These non-SDS isolates were F. solani, F. merismoides, a Pythium sp., and a Paecilomyces sp. All of these isolates produced some root rot, but significantly less than the F. solani f. sp. glycines reference isolate. The results of this study established that soil populations of F. solani f. sp. glycines can be accurately enumerated using MNSM. It should be noted that, on both MNSM and PDA, F. solani f. sp. glycines can resemble some isolates of F. solani f. sp. phaseoli, so pathogenicity tests of random isolates may be required to confirm the forma specialis especially in fields that have a history of both Glycines max and Phaseolus vulgaris.

Sudden death syndrome of soybean in South America is caused by four species of Fusarium : Fusarium brasiliense sp. nov., F. cuneirostrum sp. nov., F. tucumaniae, and F. virguliforme

“…In previous reports from the United States Roy 1997a;Rupe et al 2001;Cho et al 2001), difference in pathogenicity among soybean SDS-causing isolates were reported under the name of F. solani f. sp. glycines.…”

Comparison of inoculation methods for characterizing relative aggressiveness of two soybean sudden-death syndrome pathogens, Fusarium virguliforme and F. tucumaniae

“…Although the underlying basis for this difference is unknown, one hypothesis might be due to sensitivity of symptomology to subtle differences in the environmental conditions when the experiments were conducted in the spring and early summer Rupe et al, 1996;Farias Neto et al, 2008). Regardless of the inoculation method used, most studies have reported a difference in aggressiveness of F. virguliforme isolates Cho et al, 2001;Rupe et al, 2001;Mueller et al, 2002a;Li et al, 2009). It is worth mentioning that disease severity has been evaluated using culture filtrates of F. virguliforme as well as a cut-seedling assay (Jin et al, 1996;Huang & Hartman, 1998;Li et al, 1999).…”

“…Even when cultivars are screened in artificially infested soil, disease incidence is unpredictable due to the sensitivity of symptomology to environmental factors Rupe et al, 1996;Farias Neto et al, 2008). Methods for assessing aggressiveness (i.e., amount of disease induced) in greenhouse studies include using soil infestation by growing the pathogen on sorghum grain (Hartman et al, 1997Huang & Hartman, 1998;Cho et al, 2001;Rupe et al, 2001;Mueller et al, 2002aMueller et al, , 2002bMueller et al, , 2003Aoki et al, 2005;Farias Neto et al, 2008;Franco et al, 2009), oat seeds (Scherm & Yang, 1996), sand-cornmeal Gray et al, 1999;Njiti et al, 2001), culture filtrates (Jin et al, 1996;Li et al, 1999), inoculation via a toothpick method Arruda et al, 2005), colonized agar plugs (Rupe, 1989), a detached leaf method (Franco et al, 2009) and conidial suspensions (Rupe et al, 1996;Njiti et al, 2001). Development of an accurate disease scoring method for screening resistance to these pathogens, in a rapid and uniform way in the greenhouse, is crucial for developing soybean cultivars with broadbased resistance to the SDS pathogens.…”

Morphological and molecular characterization of the sudden‐death syndrome pathogen of soybean in Brazil

“…Initial studies indicated that the SDS causal agent was a strain of Fusarium solani (Rupe & Weidemann, 1986;Rupe, 1987;Killebrew et al ., 1988;Roy et al ., 1989). The characteristics of this pathogen comprised a blue pigmentation and slow growth on potato dextrose agar (PDA), modified Nash & Snyder medium and V8 (Roy et al ., 1989;Cho et al ., 2001). Other morphological features which have typically been used for discrimination from other forms of F. solani include: the absence of microconidia; macroconidia 30 -60 × 6-8 µ m and mostly triseptate, originating from monophialides in simple or branched conidiophores; and the presence of chlamydospores produced from conidia or hyphae (Roy et al ., 1997;Li et al ., 1998;Rupe, 1999).…”