Root Rot of Dry Edible Bean Caused by Fusarium graminearum

Fusarium graminearum was identified as a root pathogen of a diverse array of dry edible bean genotypes under both field and greenhouse conditions in North Dakota. In comparisons under controlled conditions, root rot caused by F. graminearum was equal or greater than that caused by F. solani f. sp. phaseoli. Out of eleven dry bean genotypes evaluated in controlled conditions, Eclipse, VAX 3, and T-39 had the lowest root rot severity values for both F. graminearum and F. solani f. sp. phaseoli. A significant and positive correlation between genotype response to F. graminearum and F. solani f. sp. phaseoli indicates that genetic resistance to both pathogens may be related. Introduction Fusarium root rot is one of the major yield-limiting diseases of dry edible bean (Phaseolus vulgaris L.) in North Dakota and Minnesota production regions (13). More dry edible beans are produced in North Dakota and Minnesota than in any other region in the United States. Dry edible bean market classes grown in North Dakota and Minnesota include black, great northern, red kidney, navy, pinto, and a few other specialty classes. Previously, most Fusarium root rot of dry edible bean was considered to be caused by Fusarium solani (Mart.) Sacc. f. sp. phaseoli W.C. Snyder & H.N. Hansen (Fsp) (6). Recently, several reports of F. graminearum Schwabe (Fg) causing disease on below-ground tissues of dicotyledonous plants have been published (1,4,5,7,8,12,17). In North Dakota and Minnesota, Fg is very common and causes Fusarium head blight on barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) (16). The objectives of our research were to: (i) determine if a pathogenic association between Fg and dry edible bean occurs under natural field conditions; (ii) test Fg isolates from dry edible bean to compare aggressiveness with an Fsp isolate; and (iii) evaluate dry edible bean genotypes for resistance to root rot caused by Fg and Fsp. Dry Edible Bean Genotype Field Study General methods. Dry edible bean genotypes Eclipse (black bean), Maverick (pinto bean), Norstar (navy bean), Red Hawk (dark red kidney bean), and Rojo Chiquito (small red bean) were planted into a field in Fargo, ND, at the North Dakota State University Agricultural Experiment Station on 1 June 2006 into a Fargo clay soil (fine, smectitic, frigid Typic Epiaquerts). Planting depth was approximately 4 cm. Each plot was 4 rows wide (38 cm centers) and 4.6 m long. The field had been planted to spring wheat and used for Fusarium head blight fungicide research trials in 2005. The experimental design was a randomized complete block (RCB) with 4 replications. The field study originally was designed to be a dry bean cultivar yield and performance test, but served as a source of Fusarium isolates for further studies and as a preliminary trial in which dry bean genotypes could be compared for root rot severity. 25 April 2011 Plant Health Progress Fusarium graminearum from roots. Fusarium pathogens causing root rot were identified through isolations as follows. On 29 June, twenty plants per plot were arbitrarily chosen and dug, collected, and taken to the laboratory. Roots were removed from the plants, washed to remove soil, and cut into sections approximately 1 cm in length. Root pieces from all plants were combined, and 50 root pieces were selected arbitrarily, surface sterilized in a 0.5% solution of NaOCl for 2 min, rinsed in sterile distilled water three times, and placed into petri dishes containing Komada’s medium (14). All Fusarium isolates that grew from the plated roots appeared to be Fg according to morphological characters (15). To confirm the identification, six isolates were sent to the Fusarium Research Center (Penn State University, University Park, PA). All six isolates were confirmed as Fg through morphological characters by the Fusarium Research Center, and for five of the isolates, partial translation elongation factor 1-alpha sequences were generated for identification as described by Geiser et al. (11). The sequences from the five isolates were 99% to 100% identical to Fg. The mean number of plated root pieces (out of 50) in which Fg was isolated from each genotype is shown in Table 1. These data were analyzed using the general linear models procedure (PROC GLM) of SAS (Version 9.2, SAS Institute Inc., Cary, NC), and means were compared using Fisher’s protected least signficant difference (LSD), where α = 0.05. Genotypes did not significantly differ for the number of roots in which Fg was recovered. Table 1. Root rot severity and number of roots in which Fusarium graminearum was isolated from different dry bean genotypes growing in a field located in Fargo, ND. x Means within a column followed by the same letter are not significantly different according to Fisher’s protected least significant difference (α = 0.05). y Represents the mean number of root pieces that yielded F. graminearum colonies. A total of 50 root pieces collected from each plot were plated onto Komada’s medium. Root rot severities. Dry edible bean genotypes were assessed for root rot severity in the field. On 12 August, ten plants from each plot were dug, roots were washed to remove soil, and roots were rated for root rot severity using a 1 to 7 scale described by Schneider and Kelly (18), where: 1 = healthy roots with no discoloration of root or hypocotyl and no reduction in root mass; 2 = 0.1 to 0.2 cm small reddish brown lesions at the base of the hypocotyl, with normal root mass and size; 3 = increase in intensity and size and coalescing of localized root/hypocotyl lesions approximately 180° around the stem, with lesions from 0.5 to 1 cm and 10 to 20% root discoloration but no reduction in root mass size; 4 = increase in intensity of discoloration and size of hypocotyl lesions, with lesions extending and completely encircling the stem, 5 to 10% root mass reduction, and 95% of the roots discolored; 5 = increasingly discolored and extended hypocotyl lesions, with 100% of the roots intensely reddish-brown and 20 to 50% root mass reduction; 6 = hypocotyl lesions encircling the stem extending up to 2 cm, intense root mass discoloration, and 50 to 80% root mass reduction; and 7 = pithy or hollow hypocotyl with very extended lesions, 80 to 100% root mass reduction, and functionally dead. Data were analyzed using PROC GLM in SAS, and means were compared using Fisher’s protected LSD, where α = 0.05. Genotype Root rot severity (1 to 7)x Root pieces with F. graminearum (no.)xy Red Hawk 4.3 a 6.3 a Rojo Chiquito 4.3 a 3.0 a Maverick 3.5 b 6.8 a