EFFECT OF TEMPERATURE AND AIR HUMIDITY ON MULTIPLICATION OF XANTHOMONAS-CAMPESTRIS PV. UNDULOSA AND SYMPTOM EXPRESSION IN SUSCEPTIBLE AND FIELD-TOLERANT WHEAT GENOTYPES

Multiplication of Xanthomonas campestris pv. undulosa in the leaf tis sues of susceptible and field-tolerant wheat genotypes was determined at the seedling stage, under different incubation conditions of temper ature and air humidity. Dry air conditions did not hamper the multipli cation of the bacteria in the leaf tissues, and temperature had a dire ct effect on the population trend. Symptoms were not observed at 15 de grees C. The occurrence of lesions coincided with populations of bacte ria that had reached an estimated threshold of 10(8) colony forming un its/ leaf. Differences in populations between genotypes Alondra and Pa von were only noticeable at 25 degrees C and dry air conditions, witho ut correlation to differences in symptom expression. Considering these two genotypes, resistance to bacterial leaf streak did not seem to op erate at the seedling stage when plants were inoculated by infiltratio n. In both genotypes, lesions did not extend significantly. An active mechanism stopping bacterial growth which is apparently temperature-de pendent or a saturation of the target sites was possibly involved, as shown in the different experiments by the fit-curves calculated for Al ondra and Pavon. This study provided information on the role of climat ic factors in relation to epidemics. Temperature appeared to have a ma jor effect on the disease and results confirmed that the risk of bacte rial leaf streak prevails in warmer non-traditional wheal growing area s. Several types of resistance are likely to exist in the X. c. pv. un dulosa/wheat system.