Chapter 13

Plant Disease Management for Organic Crops

Soil Solarization for Controlling Soilborne Diseases

Soil solarization is a hydrothermal process for controlling soilborne diseases in soil prior to planting crops. Effective control of soilborne plant pathogens (plant parasitic nematodes, fungi, and some bacteria) is a serious challenge to organic farmers. Crop rotation or selection of new sites can be effective for preventing soilborne some diseases. However, where susceptible crops are repeatedly planted, soilborne disease problems often develop and persist. Soil fumigants can be used to control soilborne diseases but there is a significant environmental risk, a negative impact on beneficial soil microorganisms, and generally not approved for use in organic crop production.

Principles

Soil solarization captures the radiant energy of sunlight under clear polyethylene film. The transparent film allows short-wave radiation from the sun to penetrate the plastic. Once the light passes through the plastic and is reflected from the soil, the wavelength becomes longer and cannot escape through the plastic. The trapped light facilitates heating of the soil to temperatures detrimental to most living organisms.

Site Selection and Preparation

Shady areas should be avoided because temperatures likely will not reach lethal levels. The soil to be solarized must be worked up to seed-bed condition—that is, cultivated until it's friable with no large clods or other debris on the soil surface.

Film Selection and Application

Different types of plastic sheets are available, mainly differing in their thickness (insulation) and ability to let light through (transparency). Black, opaque, or translucent plastics are not suitable for solarization because instead of letting radiation pass through and heating the underlying soil, solar energy is absorbed and radiated back into the air and only slight warming of the covered soil occurs. Thin, transparent plastic sheets appear to achieve the best results—between 0.5 to 4.0 mil. Thinner (0.5 to 1 mil) films conduct more heat, but are more susceptible to tearing and have a shorter life than thicker films. Often growers will use two layers of thin plastic sheeting separated by a thin insulating layer of air increases soil temperatures and the overall effectiveness of a solarization treatment. Agricultural films for commercial application are often treated with ultraviolet (UV) inhibitors to delay film breakdown.

Considerations

The main disadvantage of solarization is its potential negative impact on beneficial soil microorganisms since they will meet the same fate as their harmful counterparts. But recovery is usually attained quickly through rapid recolonization because beneficial microorganisms appear better suited to utilize substrate and nutrients freed by solarization than their harmful counterparts.

Effect on Soilborne Pathogens

Soil solarization is effective against fungal pathogens such as Verticillium spp. (wilt), Fusarium spp. (several diseases), and Phytophthora cinnamomi (Phytophthora root rot), and bacterial pathogens such as Streptomyces scabies (potato scab), Agrobacterium tumefaciens (crown gall), and Clavibacter michiganensis (tomato canker).

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