Morphologic, Molecular, and Pathogenic Characterization of Diaporthe phaseolorum Variability in the Core Soybean-Producing Area of Argentina

Abstract

Isolates of the Diaporthe/Phomopsis (D/P) complex were collected in the main soybean producing area of Argentina during the 1996-97, 1997-98, and 1998-99 growing seasons. Twenty-three morphologic characters related to type of colonies, stroma, pycnidia and conidia, presence of perithecia, and asci length were studied by principal component analysis (PCA). Genomic DNA were analyzed by the random amplified polymorphic DNA (RAPD) technique. From both studies, 18 isolates were identified as D/P complex and grouped in four major taxa: (i) Diaporthe phaseolorum var. meridionalis, (ii) D. phaseolorum var. caulivora, (iii) D. phaseolorum var. sojae, and (iv) Phomopsis longicolla. In addition to distinguishing interspecific and intraspecific variability, molecular markers allowed the detection of differences among isolates within the same variety. Pathogenicity was assayed in the greenhouse, by the toothpick method, inoculating the D/P isolates to soybean genotypes carrying different resistance genes (Rdc1, Rdc2, Rdc3, and Rdc4) against soybean stem canker (SSC). Pathogenic analysis distinguished two main groups: (i) the SSC-producing isolates, including D. phaseolorum var. meridionalis and D. phaseolorum var. caulivora, and (ii) the non-SSC-producing isolates, including D. phaseolorum var. sojae and P. longicolla. Cultivar RA-702 (susceptible control) was compatible with both D. phaseolorum var. meridionalis and D. phaseolorum var. caulivora isolates; meanwhile, Tracy-M (Rdc1 and Rdc 2 genes) was incompatible with D. phaseolorum var. meridionalis but compatible with D. phaseolorum var. caulivora isolates. The fact that Rdc1 and Rdc2 together (as in Tracy-M) confer an almost immune reaction to all assayed isolates of D. phaseolorum var. meridionalis but were ineffective against the D. phaseolorum var. caulivora isolates evaluated suggests that the virulence or avirulence genes in D. phaseolorum var. meridionalis and D. phaseolorum var. caulivora are different. Moreover, physiological races of D. phaseolorum var. meridionalis were detected by using differential soybean genotypes carrying distinct single Rdc genes. As far as we know, this is the first report on the existence of physiological races of D. phaseolorum var. meridionalis in South America. Selective pressure due to deployment of resistant host cultivars may have changed the frequency of the virulence or avirulence genes within the population of D. phaseolorum var. meridionalis. On the whole, our results show that pathogenic variability of D. phaseolorum in the core soybean-producing area of Argentina is higher than previously recognized.

Response of the edamame edible soybean germplasm to Diaporthe phaseolorum, causal agent of soybean stem canker, in Argentina

“…Rdc3, present in cultivar Crockett, and Rdc4 in cultivar Dowling (Bowers et al, 1993) and in cultivar Hutcheson (Tyler, 1996 (Pioli et al, 2001), and the resistance genes were re-named as Rdm1 to Rdm4 based on published evidence that the resistance genes to Dpm were not effective against Dpc isolates (Pioli et al, 2003;Pioli & Morandi, 2006;Chiesa et al, 2009). The SSC disease…”

“…Dpm and Dpc local isolates were collected from different agro-ecological regions and were morphologically and molecularly identified (Pioli et al, 2003) according to the techniques proposed by Fernández & Hanlin (1996). The isolates identified by our group were deposited in the Culture Collection of CEREMIC (Centro de Referencia de Micología, Fac.…”

“…The isolates were cultured on potato glucose agar medium (PGA) 2% acidified with lactic acid 0.2%, and incubated at 26 ± 1ºC in darkness. When developed, previously to the experiment, isolates were inoculated on susceptible control (cultivar RA702) and re-isolated from it to verify their virulence condition (Pioli et al, 2003).…”

“…Control seedlings were similarly wounded but no mycelia were applied. For the first 72 h after inoculation, For the first 72 h after inoculation, inoculated and control seedlings were kept in high humidity atmosphere using a transparent plastic coverage in natural photoperiodic conditions (Pioli et al, 2003). Meanwhile the (Pioli et al, 2003).…”

“…To calculate the percentage dead plant (%DP), a value of 1 was assigned to each dead plant, a value of 0.5 to each plant displaying stem canker symptoms or foliar discoloration and a value of 0 for healthy plant. The % DP was calculated as the proportion of plants that were either dead or showed any SSC symptoms out of the total number of plants observed in each interaction (Pioli et al, 2003). According to the %DP, plant-pathogen interactions were classified as resistant = 0 to 14.9% DP; moderately susceptible = 15 to 49.9% DP; susceptible = 50 to 84.9% DP and highly susceptible = 85 to 100% DP.…”