Effects of Broccoli Rotation on Lettuce Drop Caused by Sclerotinia minor and on the Population Density of Sclerotia in Soil

Abstract

Field experiments were conducted at Spence Road site and at the Hartnell College East Campus site in Salinas, CA, to determine the effects of crop rotation with broccoli or a fallow period on lettuce drop caused by Sclerotinia minor and the density of pathogen sclerotia in the soil. Treatments at the Spence Road site with low inoculum density (<7 sclerotia per 100 cm3 of soil) distributed randomly included: successive crops of lettuce (LLL), lettuce rotated with broccoli (LBL), and lettuce followed by a fallow period (LFL). Treatments at the Hartnell site with high inoculum density (>7 sclerotia per 100 cm3 of soil) distributed uniformly included: continuous lettuce (LLLL), broccoli-lettuce-broccoli-lettuce (BLBL), broccoli-broccoli-lettuce-lettuce (BBLL), and fallow-lettuce-fallow-lettuce (FLFL). At the Spence Road site, continuous lettuce did not increase lettuce drop incidence for at least 2 years, although an increase in soilborne sclerotia was observed annually but was below the threshold at which a correlation between inoculum density and disease incidence is observed. Rotation with broccoli resulted in small reductions in disease incidence only in the first year. The density of sclerotia was lowest in the LFL treatment, and the highest in the LLL. At the Hartnell site, rotation with broccoli significantly reduced both sclerotia and lettuce drop incidence. The number of broccoli crops rather than the sequence of lettuce rotations with broccoli was critical for reducing the numbers of S. minor sclerotia in soil. Fallowing after a lettuce crop resulted in marginal reductions in sclerotia and lettuce drop incidence. Viability of recovered sclerotia was not significantly different between treatments, although differences between seasons were detected. Results suggest that rotations with broccoli can be a practical lettuce drop management strategy.

Managing for soil health can suppress pests

“…Incorporating cover crops can also produce bioactive compounds that behave similarly to chemical soil fumigants (biofumigation), causing changes in microbial communities and suppressing pests (Hao et al 2003;Koike and Subbarao 2000). In peaches, UC and USDA-ARS researchers found that short preplant rotations of sudangrass helped manage Prunus replant disease, improving calculated profits even when fruit prices were low .…”

Biotic interactions, ecological knowledge and agriculture

“…that produce glucosinalates, sudan grass and cereal rye (SAN 1998). Biofumigation with broccoli residue incorporation suppresses Sclerotinia minor in lettuce (Hao et al 2003) and Verticillium wilt in cauliflower when disease pressure is moderate. One reason for the variable performance of Brassica spp.…”

Biomass crops can be used for biological disinfestation and remediation of soils and water

“…But additional chemical compounds arising from nonglucosinolate pathways -including aldehydes, acids and other sulfur-and nitrogen-containing compounds released during plant growth or decomposition in soilalso have significant pesticidal activity (Kelly and Baker 1990;Gamliel and Stapleton 1993;Bending and Lincoln 1999). Furthermore, besides chemical activity, alterations in microbial activity (Gamliel and Stapleton 1993) that are deleterious to pest organisms (Hao et al 2003) have also been associated with Brassica-mediated soil disinfestation. In California, published results on biofumigation with Brassica spp.…”

“…Experimental work in the Salinas Valley reported that broccoli (B. oleracea var. italica) rotations were effective in controlling certain soilborne fungal pathogens, such as Verticillium and Sclerotinia, in vegetable crop rotations (Koike and Subbarao 2000;Hao et al 2003). In contrast, cropping and subsequent soil incorporation of rape/canola (B. napus) and certain mustards (B. juncea and Sinapis alba) had no significant effects on the soilborne pests of processing tomatoes in the Sacramento Valley (Hartz et al 2005).…”