Brown leaf spot of peanut: studies on morphology, physiology of the pathogen and screening for disease resistance

Brown leaf spot of peanut caused by Cercospora arachidicola Hori. Screening for brown leaf spot resistance of peanut in the field and greenhouse using 29 peanut varieties revealed that the high resistance accession were KKU NO.101 and Robut 33-1 x NCAc 17090 (71-1-22) (disease score 3.38. NCAc 17090 accession was moderate resistance (disease scores 6.11) in the field but in the greenhouse it was susceptible (size of lesion 5.33, No.conidia/lesion 12.67). While RLRs 10 accession was moderate resistance in the greenhouse but in the field it was susceptible. The others moderate resistance accession were NCAc 17090 x RCM 387 (21-6-10), NCAc 17127 x RCM 387 (53T-2-11), Robut 33-1 x NCAc 17127 (54-2-13) and NCAc 17090 (31-9-16) (disease score 4.23-5.37). Observation made under compound and scanning electron microscopes (SEM) on morphology of C. arachidicola revealed that the fungus has a characteristic as follows: conidia sybhyaline to pale olivaceous brown, obclavate, straight to mildly curved, base truncate, tip obtuse size 1.25-5 x32.5-125 micro m conidiophores subgeniculate, tip round, the scars marking the point of attachment of the conidia are plainly visible, not branched, size 2.50-7.50x37.5-112.5 micro m. Comparison of the histopathological properties of the fungus, in refering to host-parasite interaction system revealed that 8 days after inoculation, penetration and infection took place directly into epidermal cell of susceptible host while germ tubes on resistant host ramified epidermal cell but penetration was not observed. After inoculation for 30 days, germ tubes were observed in the resistant host with a few mycelia that mostly grew as intercellular of vascular bundles and lower epidermis. In the susceptible host, germ tubes grew to form mycelia and became to be stroma. Susceptible tissues collapsed and inveded cells died quickly. Stroma, conidiophore and condidia were formed abundantly on host surface.