Assessment of Resistance in Lettuce (Lactuca sativa L.) to Mycelial and Ascospore Infection by Sclerotinia minor Jagger and S. sclerotiorum (Lib.) de Bary

Abstract

<jats:p>Lettuce drop caused by <jats:italic>Sclerotinia</jats:italic> spp. is an economically important disease of lettuce (<jats:italic>Lactuca sativa</jats:italic> L.), and cultivars with resistance to mycelial infection by <jats:italic>Sclerotinia sclerotiorum</jats:italic> (Lib.) de Bary and <jats:italic>S. minor</jats:italic> Jagger as well as to <jats:italic>S. sclerotiorum</jats:italic> ascospores are needed. Assessing resistance in field experiments can be complicated by fast bolting or small stature lettuce lines that may escape rather than resist the pathogens. Therefore, methods to select resistant lines from morphologically variable populations are needed. We used <jats:italic>S. sclerotiorum</jats:italic> and <jats:italic>S. minor</jats:italic>-infested field experiments, regression analysis, field experiments with artificially high plant densities, and <jats:italic>S. sclerotiorum</jats:italic> ascospore inoculations to identify lettuce lines with resistance to both pathogens. Three replicated experiments in <jats:italic>S. sclerotiorum</jats:italic>-infested fields were conducted in Yuma, AZ, and three replicated experiments in a <jats:italic>S. minor</jats:italic>-infested field were conducted in Salinas, CA, using diverse populations of iceberg, romaine, leaf, butterhead, Latin, oilseed lettuce, and wild relatives of lettuces; and genetic variation for the incidence of lettuce drop from mycelial infections was identified. In two <jats:italic>S. minor</jats:italic> field experiments, a quadratic regression model was developed that related rapid bolting with reduced lettuce drop. Regression residuals were calculated, and eight cultivars or PIs had negative residuals in two independent field experiments, indicating higher resistance than predicted by their rate of bolting. Eruption, a small-statured Latin cultivar, had significantly lower disease levels than susceptible cultivars in experiments with high plant densities, indicating that its small size did not facilitate disease escape. Ascospore inoculations confirmed resistance in ‘Eruption’ and <jats:italic>L. virosa</jats:italic> SAL012, whereas the oilseed lettuce PI 251246 may have partial resistance to infection. These lines will likely be useful for development of <jats:italic>Sclerotinia</jats:italic> spp.-resistant lettuce cultivars.</jats:p>

A single recessive gene conferring short leaves in romaine × Latin type lettuce (Lactuca sativa L.) crosses, and its effect on plant morphology and resistance to lettuce drop caused by Sclerotinia minor Jagger

“…Lettuce drop can be caused by two fungal species, S. minor and S. sclerotiorum (Subbarao 1998). While S. minor predominates in the lettuce growing regions in coastal valleys of California, ÔEruptionÕ has demonstrated resistance to mycelial plant crown infections of both species as well as the aerial ascospore phase of S. sclerotiorum (Hayes et al 2010). We did not test the effect of sl1 on resistance to S. sclerotiorum, or on resistance to X. campestris pv.…”

“…This site has a history of S. minor experiments beginning in 2007, when it was fumigated with 420 kg/ha of Tri-Con 57/43 Methyl-Bromide/Chloropicrin (Trical Inc, Hollister, CA, USA) prior to all experiments. To annually infest the field, sclerotia of S. minor isolates (BM001, BM005, BM007 and BM010) were produced and laid into the seed line using the method of Hayes et al (2010) at a rate of approximately 360 sclerotia/m in the spring immediately prior to planting. The subsequent summer experiment used the residual inoculum (sclerotia) in the soil augmented by the plants that died from lettuce drop during the spring experiment.…”

“…2006). ‘Eruption’, which the authors described as a red Latin, is reported as resistant to lettuce drop caused by Sclerotinia minor and Sclerotinia sclerotiorum (Hayes et al. 2010).…”

“…ÔPavaneÕ and ÔLittle GemÕ have been described as resistant to big vein (Bos andHuijberts 1990, Hayes et al 2006). ÔEruptionÕ, which the authors described as a red Latin, is reported as resistant to lettuce drop caused by Sclerotinia minor and Sclerotinia sclerotiorum (Hayes et al 2010). All of the Latin cultivars mentioned here, as well as others, are resistant to race 1 isolates of Verticillium dahliae .…”

Effect of a High Tunnel, Organic Cropping System on Lettuce Diseases in Western Washington

“…tipburn, a calcium deficiency, reduces lettuce quality and marketability when inner leaves (which cannot be trimmed or discarded) are affected (4). Few lettuce cultivars have exhibited tolerance to B. cinerea (5) while only physiological and partial resistance to S. sclerotiorum has been demonstrated (9,18). Continued breeding efforts are needed before lettuce cultivars resistant to these necrotrophic fungi are available commercially (18).…”