Toxigenic capacity and trichothecene production by Fusarium graminearum isolates from Argentina and their relationship with aggressiveness and fungal expansion in the wheat spike.

At least 20 epidemics of Fusarium head blight (FHB) of wheat have been registered in the last 50 years in Argentina, with variable intensity. Damage induced by the disease is further aggravated by the presence of mycotoxins in affected grains that may cause health problems to humans and animals. The trichothecene chemotype was analyzed for 112 isolates of Fusarium graminearum from Argentina by polymerase chain reaction and two field trials were conducted to study the aggressiveness of a subsample of 14 representative isolates and to analyze deoxynivalenol (DON) production in planta and in vitro. All isolates belonged to the 15-acetyl-DON chemotype. Significant differences were observed in both the symptom severity induced in wheat spikes and the in vivo DON production, and a close correlation was found between these two variables. However, in vitro toxigenic potential was not correlated with the capacity of F. graminearum isolates to produce DON under natural conditions. The progress of infection in the rachis of inoculated wheat spikes was analyzed and the pathogen presence verified in both symptomatic and symptomless spikes. Even isolates with a limited capacity to induce symptoms were able to colonize the vascular tissue and to produce considerable amounts of DON in planta.

Fusarium Wilt of Lettuce Caused by Fusarium oxysporum f. sp. lactucae in Argentina.

The green belt area surrounding the city of La Plata, Argentina, produces more than 70% (around 280 ha) of the lettuce (Lactuca sativa L.) cultivated under greenhouse for fresh consumption in the country. In February 2011, April 2012, and December 2013, butterhead lettuce plants from cv. Lores showing wilt and stunted growth symptoms, red-to-brown discoloration of vascular tissues, and yellow leaves were found in greenhouses in La Plata. Sections of tap root, crown and stem from symptomatic plants showing dark-brown streaking of the vascular tissue were surface sterilized and isolations were made. A total of 21 monosporic isolates obtained from different lettuce production fields were identified as Fusarium oxysporum Schltdl. based on morphological characteristics (2). Vegetative compatibility group (VCG) analysis (1) was conducted on 11 of these isolates and all isolates belonged to the same VCG. To fulfil Koch's postulates, two pathogenicity tests were conducted with these 11 isolates in winter (July 2012) and summer (December 2013). Healthy 20-day-old butterhead lettuce seedlings of two cultivars (Reina de Mayo and Lores in the winter and summer tests, respectively) were inoculated by dipping the roots of each plant in a spore suspension (~3 × 105 CFU ml-1), planted in 1-liter pots containing autoclaved soil and grown in a greenhouse with only natural daylight. Control treatments were prepared by dipping the seedling roots in sterilized distilled water. All inoculated plants showed wilt symptoms 15 to 20 days after inoculation (dai) and 45 to 50 dai in the summer and winter pathogenicity tests, respectively. The delay in the appearance of symptoms observed during the winter test is consistent with the effect of planting date on the development and final incidence of Fusarium wilt of lettuce reported by Matheron et al. (3). No symptoms were observed in control treatments. F. oxysporum was re-isolated from vascular tissues of the stems of symptomatic plants and the formae speciales lactucae J.C. Hubb. & Gerik was confirmed by PCR using the specific primer pair GYCF1 and R943 (4). The identification of only one VCG for the tested isolates agrees with the hypothesis of seed transmission of the pathogen, which might explain the dissemination of Fusarium wilt of lettuce in geographically distant areas (2). Studies are being carried out to determine the race of these Argentinian isolates of F. oxysporum f. sp. lactucae. To our knowledge, this is the first report of F. oxysporum f. sp. lactucae infecting lettuce in Argentina. References: (1) J. C. Correll et al. Phytopathology 77:1640, 1987. (2) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006. (3) M. E. Matheron et al. Plant Dis. 89:565, 2005. (4) G. C. Y. Mbofung and B. M. Pryor. Plant Dis. 94:860, 2010.

First Report of Fusarium Wilt of Basil Caused by Fusarium oxysporum f. sp. basilici in Argentina.

Annually, ~20 ha of sweet basil (Ocimum basilicum L.) are cultivated in greenhouses in the green belt area surrounding La Plata, Argentina, mainly for fresh consumption. In 2004 to 2007, basil plants of cv. Genovese showed wilt symptoms, necrosis of leaves and stems, asymmetrical growth, and discolored vascular tissue in greenhouses in La Plata. In 2007, the same symptoms were observed on plants of cv. Morada grown from seeds that were produced in Italy. Isolations were completed from root, crown, and stem sections of diseased plants of cv. Genovese from three greenhouses in 2004 to 2007, and from commercial seeds, stem sections, flowers, and seeds of diseased plants of cv. Morada in 2007. Seeds and portions of symptomatic tissues were surface-disinfested with 0.5% NaOCl for 1 min, rinsed in sterilized distilled water, air dried, and plated on 2% potato dextrose agar (PDA). Twenty-seven isolates were identified as Fusarium oxysporum Schltdl. based on morphological characteristics (4), and the species identification confirmed by PCR assay using a F. oxysporum f. sp. basilici-specific primer pair, Bik 1 and Bik 2 (1). Vegetative compatibility groups (VCGs) were determined for the 27 isolates through complementation of nitrate-nonutilizing mutants generated from these isolates (2) and paired with two Italian tester strains from an international collection (PVS-Fu 220 and PVS-Fu 125, provided by V. Balmas, Univeristà degli Studi di Sassari, Italy). All 27 isolates from Argentina belonged to VCG 0200. This is a unique VCG for F. oxysporum f. sp. basilici and has been identified in Israeli, American, and Italian isolates of the fungus (3). To fulfill Koch's postulates, pathogenicity tests were conducted with 12 isolates selected to reflect the multiple sources of fungal recovery, including root, crown, and stem sections, and leaves of diseased plants of cv. Genovese and commercial seeds, stem sections, flowers, and seeds of cv. Morada. Isolates were each grown on moistened (40% w/w), autoclaved, polished rice for 10 days, dried, and ground in a grinder. The number of CFU/g rice was determined by serial dilution plating onto PDA plates. The inoculum was added to autoclaved soil at 104 CFU/g dry soil. For each isolate, 8 healthy basil seedlings of each of cvs. Genovese and Morada were planted in pots, each containing 1 liter of inoculated soil. The control treatment consisted of 8 basil seedlings of each of the same cultivars planted in autoclaved soil mixed with sterilized, ground, polished rice. Plants were grown in a greenhouse with natural daylight for 45 to 50 days after inoculation. All inoculated plants showed the same symptoms described for the original basil plants. No symptoms were observed on the control plants. F. oxysporum f. sp. basilici was re-isolated from the vascular tissue of stems of symptomatic plants but not from control plants, and species identification confirmed by PCR assay as previously described. The presence of the pathogen was verified in the seed lot produced in Italy, suggesting that this could have been a source of inoculum that introduced the pathogen into La Plata, Argentina, as supported by the hypothesis that infested seed resulted in spread of a clonal population of F. oxysporum f. sp. basilici internationally (1). To our knowledge, this is the first report of F. oxysporum f. sp. basilici infecting sweet basil in Argentina. References: (1) A. Chiocchetti et al. Plant Dis. 85:607, 2001. (2) J. C. Correll et al. Phytopathology 77:1640, 1987. (3) A. Garibaldi et al. Plant Dis. 81:124, 1997. (4) J. F. Leslie and B. A. Summerell. The Fusarium Laboratory Manual. Blackwell Publishing, Ames, IA, 2006.