Chlamydospore germination and Fusarium wilt of banana plantlets in suppressive and conducive soils are affected by physical and chemical factors

To determine the factors affecting the germination, early germ-tube growth (collectively called 'germination') of chlamydospores of Fusarium oxysporum f. sp. cubense and the severity of Fusarium wilt in banana plantlets, we v aried chemical and physical factors in a suppressive and conducive soil. So il temperature (4-40 degrees C), water content (40-80% field capacity), and pH (4-10) were varied, and various amounts of CaCO3, Ca(OH)(2) and Fe-EDDH A were added ro the soil. The suppressive soil had higher populations of ac tinomycetes and bacteria than the conducive soil that had higher population s of filamentous fungi and yeasts. The suppressive soil reduced germination of chlamydospores by 41 +/- 4%, and more than halved disease severity in b anana plantlets (Musa spp., AAA, Cavendish subgroup, cv. Williams). Soil wa ter content, from wilting point to 80% field capacity, had little effect on germination of chlamydospores in either soil. In contrast, a water content of 40% field capacity promoted disease severity in both soils, compared wi th wetter soil. Increasing the temperature from 20 to 30 degrees C in the c onducive soil increased chlamydospore germination by 1.9 fold, but in the s uppressive soil it had a much less effect (1.2 fold increase). This indicat ed that a physical process, such as diffusion, is likely to limit germinati on in the suppressive soil. An increase in temperature from 24 to 34 degree s C increased disease severity in both suppressive and conducive soils. At 14 degrees C no wilt symptoms were evident but the pathogen was recovered f rom within the plant. Chlamydospore germination was greatest in both soils at their natural pH of 8. A similar pattern existed for disease severity in the conducive soil. However, in the suppressive soil pH had little effect on disease severity. Adding CaCO3, Ca(OH)(2), CaSO4 or Fe-EDDHA to the soil reduced germination and disease severity by one-third to one-half in both soils. Smaller amounts had the greatest effect and the amounts of Ca compou nds used were insufficient to change soil pH. The effects of manipulating s oil pH, Ca and Fe content, temperature and water content on disease express ion in the field need to be demonstrated. (C) 1999 Elsevier Science Ltd. Al l rights reserved.