by G. Gilardi, S. Franco Ortega, S. Demarchi, M. Pugliese, M.L. Gullino, A. Garibaldi, AGROINNOVA

 

Fusarium oxysporum f. sp. lactucae (FOL), the causal agent of lettuce Fusarium wilt, is an important disease affecting lettuce production in many countries throughout the world. It was first found in Europe in northern Italy on lettuce grown under intensive cropping systems for the ready-to eat sector in 2002.

The pathogen can be seedborne, which provides a likely mechanism for long distance dispersal. Locally, this pathogen can be moved between fields with contaminated soil on farming equipment. Once established, the pathogen is difficult to eradicate, unless soil fumigation is an option.

At present, lettuce Fusarium wilt is managed primarily by the use of resistant cultivars. However, their efficacy may be limited by the presence of four races of the pathogen. At this moment, only Race 1 is widespread worldwide and it is the only race present in Italy.

The objective of the study at AGROINNOVA was to provide information on the virulence of FOL isolates obtained from the affected lettuce plants grown in different farms with a history in lettuce production in Lombardy, Piedmont and Veneto. Experiments were carried out in greenhouse at temperatures between 26 and 30 °C by inoculating each monoconidial isolate of the pathogen on lettuce cultivars chosen among the most used in practice were the disease is present. The assay was carried out by root immersion of 25-30-days-old seedlings in each conidial suspension at 1x106 CFU/ml. The virulence of the isolates was compared with the Mya3040 FOL isolate first detected in 2002 in northern Italy. All isolates have been identified as belonging to race 1 by means of specific-PCR assays and tested on differential cultivars. Results suggest that the use of lettuce cultivars resistant to F. oxysporum f.sp. lactucae under monoculture may have favored changes in the virulence and/or pathogenicity structure in FOL populations.

 

trials lettuce  
Fig. 1. Potted trials under greenhouse conditions                                       Fig. 2. Assay to test the susceptibility of lettuce cultivars to Fusarium oxysporum f.sp. lactucae  

 

The study showed difference in virulence among FOL isolates belonging to race 1, probably due to the fact that some cultivars may have a quantitative resistance, which can be influenced by the environment. The concentration of inoculum allowed a good resolution for differentiation of degrees of susceptibility among the cultivars. Isolates belonging to the same race, coming from different geographical regions, can show differences in their ability to cause disease. These differences among the FOL isolates may also suggest that the use of lettuce cultivars resistant to F. oxysporum f.sp. lactucae under monoculture has favoured changes in the virulence and/or aggressiveness in FOL populations under field conditions.

 Other experimental trials have been carried out at AGROINNOVA to evaluate the efficacy of preventative treatments based on plant defense activator products, biocontrol agents, a microbial complex with arbuscular mycorrhizal fungi, and Brassica carinata pellets against Fusarium oxysporum f. sp. lactucae race 1 on lettuce and Fusarium oxysporum f. sp. raphani on cultivated rocket under artificially infested soil in greenhouse conditions. These products were compared with fungicides known for their ability to induce host resistance (phosethyl-Aland acibenzolar-S-methyl), and with azoxystrobin. Three and four applications of the tested products were carried out on lettuce and rocket seedlings grown in nursery conditions. The best Fusarium wilt biocontrol was obtained after four applications of Bacillus subtilis Qst713 and with the Glomas microbial complex (42 and 46.7%, efficacy, respectively). B. carinata pellets provided a consistent control when applied 14 days before transplant the rocket and lettuce plants into the infested soil.

Acibenzolar-S-methyl, applied at 0.025 g/Liter, showed a disease severity (DS) reduction in lettuce Fusarium wilt from 36 to 61% and of Fusarium wilt of rocket from 54 to 73%), thus showing statistically similar results to those of azoxystrobin, which was used as a reference (DS reduction from 59 to 65%). Although the Fusarium wilt control provided by such products was not complete in the present experimental conditions, these products can be considered interesting components for an integrated pest management of the Fusarium wilt of leafy vegetables, starting from nursey applications. Moreover, the tested BCAs could become potentially useful, especially for plant monocultures.

This study has produced new information on the effects of potassium phosphite, applied at the nursery level, on reducing lettuce and rocket Fusarium wilt. An average efficacy of 69.5% was observed for lettuce, while an average efficacy of 65.2% was observed for cultivated rocket. The good fungicidal activity of the phosphite-based product, coupled with the positive effect on plant biomass, is of special interest.