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 varied chemical and physical factors in a suppressive and conducive soil. Soil temperature (4-40 degrees C), water content (40-80% field capacity), and pH (4-10) were varied, and various amounts of CaCO(3), Ca(OH)2 and Fe-EDDHA were added to the soils. The suppressive soil had higher populations of actinomycetes and bacteria than the conducive soil that had higher populations of filamentous fungi and yeasts. The suppressive soil reduced germination of chlamydospores by 41 +/- 4%, and more than halved disease severity in banana plantlets (Musa spp., AAA, Cavendish subgroup, cv. Williams). Soil water 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 with wetter soil. Increasing the temperature from 20 to 30 degrees C in the conducive soil increased chlamydospore germination by 1.9 fold, but in the suppressive soil it had a much less effect (1.2 fold increase). This indicated that a physical process, such as diffusion, is likely to limit germination in the suppressive soil. An increase in temperature from 24 to 34 degrees C increased disease severity in both suppressive and conducive soils. At 14 degrees C no wilt symptoms were evident but the pathogen was recovered from 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 CaCO(3), Ca(OH)2, CaSO(4) 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 compounds used were insufficient to change soil pH. The effects of manipulating soil pH, Ca and Fe content, temperature and water content on disease expression in the field need to be demonstrated.