First report of Fusarium graminearum, F. asiaticum and F. cortaderiae as head blight pathogens of annual ryegrass in Brazil

Authors: MACHADO, F - Universidade Federal De Vicosa; MOLLER, P - Universidade Federal De Vicosa; NICOLLI, C - Universidade Federal De Vicosa; DEL PONTE, E - Universidade Federal De Vicosa; Ward, Todd

Submitted to: Plant Disease
Publication Type: Research Notes
Publication Acceptance Date: 5/30/2015
Publication Date: 12/21/2015
Citation: Machado, F.J., Moller, P.A., Nicolli, C.P., Del Ponte, E.M., Ward, T.J. 2015. First report of Fusarium graminearum, F. asiaticum and F. cortaderiae as head blight pathogens of annual ryegrass in Brazil. Plant Disease. 99(12):1859.

Interpretive Summary:

Technical Abstract: Members of the Fusarium graminearum species complex (FGSC) cause Fusarium head blight (FHB) of small grains and several grasses, including annual ryegrass (Lolium multiflorum Lam.), an important forage crop, but also a common weed in wheat, rice and maize agroecosystem in southern Brazil. Although its role as source of perithecial inoculum for epidemics in small grain has been demonstrated (1), the species composition and toxigenic potential of FGSC strains infecting this crop are not known. During disease surveys in wheat and rice fields conducted from 2008 to 2010, spikes of annual ryegrass from fields exhibiting high FHB incidence were randomly collected nearby rice fields at Camaquã and Cachoeirinha and nearby a wheat field at Passo Fundo, Rio Grande do Sul State, Brazil. Diseased spikelets were excised, surface-sterilized (70% EtOH for 30 s and 3% NaClO for 1.5 min) and rinsed three times in sterilized distilled water prior to transfer to potato dextrose agar (PDA) plates amended with streptomycin (40 mg/L). A total of 20 isolates, each from a different spike, were purified through monosporic culturing and stored on SNA (Spezieller Nahrstoffarmer agar) media at -20 oC. The fungus was transferred to carnation leaf agar (CLA) media and incubated during 10 days at 23°C under a 12-h light-dark cycle. The harvested conidia were falcate, straight or slightly curved, 5- to 6-septate conidia, with a slender apical cell and foot shaped basal cell, typical of F. graminearum sensu lato (Nelson et al. 1983). Seven isolates from the rice-growing region and three from the wheat-growing region were grown on PDA (potato-dextrose agar) media for mycelial production and DNA extraction using the CTAB method. The isolates were identified to species and trichothecene genotypes using a multilocus genotyping (MLGT) assay (2). Pathogenicity was evaluated by dip-inoculating spikes of potted ryegrass plants (one plant per isolate) at full flowering for 10s into a 10-ml conidial suspension (1 × 105 conidia/mL). An isolate from wheat, known as highly pathogenic to wheat, was included for comparison. Plants were covered with polyethylene bags and incubated in a chamber with a 12-h dark-light cycle at 25°C during 48 h. Then, they were moved back to the greenhouse and the disease was monitored daily. Symptomatic spikelets were collected 10 days after inoculation and the pathogen was re-isolated and paired with the original culture in a vegetative compatibility assay (3). Three species and two trichothecene genotypes were identified: The isolates from the wheat region were identified as F. graminearum (n=3) with the 15-ADON genotype, whereas the isolates from the rice-growing region were identified as F. asiaticum (n=4) and F. cortaderiae (n=3) with the NIV genotype. All were pathogenic to ryegrass at levels of aggressiveness similar to the F. graminearum isolate from wheat. Results suggest that ryegrass is an important source of inoculum for NIV-producing species that most commonly affect rice and maize, and DON-producing species that commonly affect wheat and barley crops.