Mustard Suppression of Cucurbits without Biofumigation

Biofumigation is one mechanism whereby cruciferous cover crops can suppress pests. We and others have sometimes observed suppression of cucurbits that are sown after incorporating mustard cover crops. To test whether this undesirable effect is a consequence of biofumigation or another property of mustards, we performed two tests that varied the intensity of biofumigation before sowing cucumber or pumpkin. In all cases, cucumbers were inhibited under conditions where no isothiocyanate would have been present in the soil. First, we grew Pacific Gold, a high-glucosinolate mustard, and varied the biofumigation by timing incorporation, applying jasmonic acid to raise the glucosinolate level or covering with plastic after incorporation. The inhibition of cucumber yield relative to the no-mustard control was equal in all mustard treatments (23%). Second, we raised low- and moderate-glucosinolate mustard (Tilney, Red Giant) in the spring, sowing cucumbers either immediately (higher biofumigation) or 5 d later. Cucumber yield was inhibited by 65% In the low-GS and 45% in the moderate GS; delaying planting slightly increased the inhibition. Third, a low-glucosinolate mustard (Tilney) was sown in the fall and allowed to winterkill, with two controls, bare ground and a non-biofumigant cover crop, buckwheat. The following spring, the field was sown to cucumbers. None of the treatments would expose pumpkins seedlings to a biofumigant. . The fall covers reduced cucumber yield equally (32%). In each of the three tests, the results are inconsistent with the inhibition of cucumbers being caused by biofumigation. Thus there is potential for finding ways to avoid having cover crops inhibit cucumbers while retaining the benefits of biofumigations.