Effects of lettuce cultivar mixtures, differing for resistance genes, on Bremia lactucae incidence
Résumé
Bremia lactucae, the causal agent of lettucedowny mildew,is still an important problem in lettuce
production in spite of many resistance genes used over the last 50 years in Europe. Every new gene
introduced into commercial varieties has been overcome in a few years. At INRA, new resistance genes
were found in two L. virosa accessions and introgressed into butterhead lettuce (lines named Vi). These
resistance genes are not yet introduced in commercial varieties; we would like propose a using strategy
of these genes that could increase their durability.
Which strategies might be efficient to reduce the evolution of the pathogen and ensure a durable
resistance with these genes? A lower selection pressure on B. factucae population could reduce the
speed of breakdown of the new resistance. A possibility to have a lower selection pressure on the B.
factucae populations could be mixture of two cultivars differing for resistance genes. Therefore Bremia
contamination has been studied for several winters in cultures of these resistant Vi lines mixed with a
susceptible cultivar. ln the same culture, the progression of Bremia on the susceptible plants has been
observed. The disease development has been studied after an artificial contamination with two
objectives: (i) does the pathogen overcome the resistance, (ii) is the disease seve rit y on the susceptible
cultivar acceptable for commercial use?
After eight experimentations under a polythene greenhouse, pure stands were compared to mixture in
ratio 1:3 (susceptible:resistant) after focal inoculation with a B. factucae isolate common in the region.
No sporulation has been observed on the two Vi lines issued from L. virosa.
For the vear with high inoculum pressure, the cultivar mixture did not reduce the disease seve rit y on the
susceptible cultivar (for example, in winter 2006-2007, 24 contaminated leaves per susceptible plant in
pure stand and in mixture; a disease seve rit y of 53 and 48 on the susceptible variety, respectively in
pure stand and in mixture).
For intermediate to low inoculum pressure, the disease development was slower on the susceptible
cultivar grown in mixtures, with less contaminated plants and less contaminated leaves per plant (1.1 in
pure stand vs 0.2 in mixture, in winter 2005-2006) at harvest. The disease severity on susceptible
cultivar is lower in mixture than in pure stand for example in winter 2005-2006 (index 1.3 vs 9.5); this
seve rit y was similar at the beginning of the epidemics for both treatments, but it became much higher in
pure susceptible stand on the last week before harvest.
Therefore the mixed culture could be an interesting method to reduce disease level on a cultivar
susceptible to the strain that is important in the culture and also possibly increase the durability of new
resistance by a lower selection pressure on the virulent strains. Nevertheless this cultural practice must
be adapted to the needs of growers economically and technically; it could be difficult to have an
efficient design of the mixture in a commercial culture. Some other experiments must be done in
connection with the growers and the breeders.