Determination of pathogenic variation in isariopsis griseola sacco and pseudomonas syringae pv. phaseolicola (burk., 1926) young, dye and wilkie 1978
Occurrence and extent of pathogenic variation and virulence was determined in Isariopsis griseola Sacco and Pseudomonas syringae pv. phaseolicola (Burk. 1926) Young, Dye and Wilkie 1978 using 21 and 30 isolates respectively, collected from naturally infected bean plants in important bean growing areas of Colombia. . griseola did not show any correlation between variation in conidial size and some cultural characteristics with the observed variation in pathogenicity. Conidial length of 10 isolates varied between 18-76~ with a mean of 38.5~. The width varied between 3.8 - 8.8~ with an average of 6.4~, whereas the number of septa varied between 0-7 with a mean of 3. These parameters varied significantly both within and between isolates. The optimum growth temperature of the fungus in culture media was 24 C. No difference in the amount of mycelial growth at 14, 19, and 24 C was observed between the isolates. Differences in sporulation among some isolates were significant but all except IG2 - 78 had an optimum sporulation temperature of 24 C after 10 days of incubation. IG2 - 78 produced significantly more conidia at 19 C than at 24 C. No correlation between the optimum growth and sporulation temperature of the isolates with the mean temperatures of their sources of origin was observed. The 21 isolates of griseola were differentiated into seven pathotypes based on differential reactions of 6 bean lines/cultivars, demonstrating occurrence of physiological specialization in the fungus. The reactions used, were independent of environmental temperatures under which plants were kept after inoculation. The cultivar - isolate interactions obtained implied a presence in the cultivar of specific resistance which was effective against some and not all the pathotypes. Cultivar G 2575-10P-2C was susceptible to all but no cultivar was immune to all pathotypes. Two out of the seven pathotypes induced a susceptible reaction on cultivar G 1805-lP-lC but of very low (disease index scale of 2) disease severity. Some of the cultivars considered susceptible appeared to have partial resistance, expressed as reduced, apparent infection rate (r) of angular leaf spot in the field. The isolates of Ps. syringae pv. phaseolicola were separated into race 1 and 2 on the basis of their pathogenicity on the cultivar 'Red Mexican DI 3'. Race determination was based on both leaf and pod reactions. Standard isolates of race 1 and 2 were used for comparison. Twenty Pasto isolates were classified as race 2 whereas, 7 isolates from Popayan, 1 from Palmira and 2 from Tenerife were identified as race 1. The isolates were also classified in their decreasing order of virulence by using the diameters of watersoaking lesions caused on pods during a period of 5 days on cultivars 'Seminole', 'G.N. Nebraska No.1 Sel 27' and 'Wisc HBR 72'; following inoculation using a hypodermic needle. The most virulent isolates were from Pasto; some of which were more virulent than the standard race 2 used for comparison. Isolates differentiated into race 1 and 2 were not homogenous in virulence. Some isolates belonging to race 1 were more virulent than some of the race 2 isolates or vice versa .apparently, due to either isolate and/or cultivar effect. Cultivar 'Wisc. HBR 72' had a high foliar resistance but a susceptible pod reaction (mean lesion diameter 3.1 rom; dispersion 0.5 ~ 5.3 rom) comparable to the susceptible cultivar 'Seminole' (mean lesion diameter 2.8 rom; .dispersion 0.5 - 4.8 rom). 'G.N. Nebraska No. 1 Sel 27' had a mean lesion diameter of 1.8 rom and a dispersion between 0.4 - 2.8 rom. The increasing or decreasing order of virulence was similar in all the three cultivars as was indicated by the high correlation coefficients (r = 0.86 - 0.87) between them. The present studies unequivocally show that both bean pathogens exhibit pathogenic variation. A high degree of cultivar - isolate specificity was observed with I. griseola. Specificity was also shown with isolates of Ps syringae pv. phaseolicola when using 'Red Mexican UI 3'. However, virulence of the bacterial isolates varied continuously in a manner which nullify the present race categorization. The practical importance of these findings are that they provide basic information about the pathogens ,that must be considered before starting a program to either incorporate or develop resistance against angular leaf spot and halo blight in P. vulgaris. The presence in some bean cultivars of pathotype specific resistance and in others partial resistance characterized by reduced rates of disease increase, indicate the available possibilities of recombining resistance aqainst; 1 griseola. To grade pathogenicity and virulence of isolates of Ps. syringae pv aseolicola, leaf and pod reactions of various bean cultivars, and not just one, must be based on. In turn, evaluation and improvement of resistance of bean cultivars against the bacterium should consider both the leaf and pod components of the plant.