A survey of Sigatoka leaf disease (Mycosphaerella musicola Leach) of banana and soil calcium levels in North Queensland

Annual averages of lesion development of yellow Sigatoka caused by Mycosphaerella musicola were calculated from surveys conducted at 14-day intervals for 57 banana sites in the North Queensland production region situated in the wet tropics between Cardwell and Innisfail. Soil up to 250 mm depth was sampled from sites between March 2000 and September 2001. Soil parameters were compared with 12 months of leaf disease data averaged centrally about the time the soil sample was taken. The 4 different formulas for calcium expression tested all proved to be significant predictors of disease levels. The strongest relationship (r^2 = 0.229) between soil calcium levels and disease levels was obtained from a formula based on an Albrecht interpretation of the soil test data, which includes a pH-derived estimation of exchangeable hydrogen in the denominator. A Chi square analysis on a model incorporating calcium and soil boron was statistically significant (P<0.001). The model combining calcium and boron levels explained more variation in disease levels than calcium alone. Variation in other factors influencing disease such as fungicide program, weather conditions, soil moisture conditions and crop stage may account for variation in disease levels at low calcium sites.