The Mother trial provides an opportunity to test a multifaceted approach to control of Verticillium in a field with a history of the disease. Although in our previous work no Verticillium wilt was detected after 5 yr of continuous strawberry when broccoli residue was incorporated, that study site had been fumigated prior to the start of our research, and levels of inocula were very low at the beginning (Muramoto et al, 2014) . Here, the site has high disease pressure (up to 30 microsclerotia/g soil have been found at CASFS; 2 /g are sufficient to induce disease). Therefore we are testing a series of “suppressive” rotations using two non-host crops, broccoli (2 and 4 year rotation) and cauliflower1 (4 year rotation), in combination with the ASD technique or the application of mustard seed cake, which also can suppress some pathogens (see Mazzola et al, 2007 Mechanism of action and efficacy of seed meal-induced pathogen suppression differ in a Brassicaceae species and time-dependent manner. Phytopathology 97:454-460). These are compared to a set of similar treatments except that broccoli is replaced with lettuce – a V. dahliae host. Lettuce is an important vegetable crop in the area, and the ability to grow it in rotation with strawberry would provide much needed flexibility of crop choice for organic farmers.
Anaerobic soil disinfestion (ASD) has been found to be an effective strategy for control of a number of soil-borne diseases including Verticillium wilt in strawberries. For more information on the technique please visit our Alternatives to soil fumigation website.
Data analysis is being completed for the assessment of disease from the mother trial and preliminary findings suggest that broccoli before strawberry reduces disease incidence relative to having lettuce as the previous crop. Also ASD showed some ability to reduce disease levels especially in the 2 year rotation, but had less effect in the 4 year rotation. However, we had problems achieving strong anaerobic conditions in the Mother trial for a variety of reasons, so the impact of ASD is likely to be less than if we had achieved the threshold level of anaerobic conditions shown previously to control Verticillium.