COMPARISON OF AERIAL CONCENTRATION, DEPOSITION, AND INFECTIOUSNESS OFCONIDIA OF PYRICULARIA-GRISEA BY SPORE-SAMPLING TECHNIQUES

In three field trials on the epidemiology of blast disease in upland r ice, different methods of sampling spores of Pyricularia grisea were c ompared to study aerial concentration, deposition, survival, and infec tiousness of conidia of the pathogen. The methods included a Burkard s pore trap, glass slides and rods, several trap-plant treatments, and l eaf prints from trap plants. The results were mostly highly intercorre lated and revealed a unimodal pattern of disease progression, with pea ks before or at the middle of the cropping seasons. The number of coni dia per square centimeter deposited on leaf surfaces as measured by le af prints made up one-fourth of the number caught with glass slides. T he amount of deposited and potentially infective spores on leaves of t rap plants of upland rice cultivar C22 incubated in a dew chamber afte r exposure in the field was 0.05-0.11, 0.42, and 0.44 of the spore den sity as measured by glass slides, leaf prints, and trap plants of the highly susceptible upland rice cultivar Co 39, respectively. The numbe r of actually infectious spores on trap plants incubated in the greenh ouse after exposure was 0.31 0.39 of total lesions observed on dew cha mber-incubated trap plants. Few spores were observed on glass rods, in dicating that sedimentation rather than impaction was the major factor for spore deposition. Nonlinear regression analysis revealed that the spore-catch results of the Burkard spore trap, glass slides, and leaf prints explained 0.58-0.65, 0.72-0.77, and 0.66 of the variability of the number of lesions on trap plants incubated in dew chambers, respe ctively. Similarly, the spore-catch results of the Burkard spore trap and the total lesions on dew chamber-incubated trap plants explained 0 .42-0.76 and 0.65-0.79 of the variability of susceptible-type lesions on trap plants incubated in greenhouse conditions, respectively. The n umber of potentially infective spores tended to increase toward an asy mptote at high inoculum concentrations. Depending on the method, the s pore-catch results were significantly affected by several weather vari ables. Estimation of deposited and infectious conidia and of inoculum potential by means of comparison of spore-sampling methods can be impr oved considerably by adjusting for the effects of these variables.