A. Sartorato

Identification of Phaeoisariopsis griseola pathotypes from five States in Brazil

Due to the increased importance of angular leaf spot of common bean (Phaseolus vulgaris) in Brazil, monitoring the pathogenic variability of its causal agent (Phaeoisariopsis griseola) is the best strategy for a breeding program aimed at developing resistant genotypes. Fifty one isolates of P. griseola collected in five Brazilian States were tested on a set of 12 international differential cultivars in the greenhouse. When inoculated plants showed symptoms but no sporulation was observed, they were transferred to a moist chamber for approximately 20-24 h. After this period of time, if no sporulation was observed, the plants were considered resistant; otherwise, they were considered susceptible. From the fifty-one tested isolates, seven different pathotypes were identified. No Andean pathotypes were identified; consequently, all isolates were classified as Middle American pathotypes. Pathotype 63-31 was the most widespread. Pathotype 63-63 overcame resistance genes present in all differential cultivars and also the resistance gene(s) present in the cultivar AND 277. This fact has important implications for breeding angular leaf spot resistance in beans, and suggests that searching for new resistance genes to angular leaf spot must be pursued.

Quantifying and modelling the mobilisation of inoculum from diseased leaves and infected defoliated tissues in epidemics of angular leaf spot of bean

Daily multiplication factor (number of daughter lesions per mother lesion per day) values were experimentally measured in four replications of a monocyclic experiment on angular leaf spot (ALS) of bean, where sources of inoculum were artificially established within a bean canopy, on the ground (defoliated infected leaves), or both. Daily multiplication factor of lesions in the canopy (DMFRc) was higher than that of infectious, defoliated tissues (DMFRd) in all replications. Both DMFRc and DMFRd were strongly reduced under dry compared to rainy conditions. Under rainy conditions for spore dispersal DMFRd was about two to three times smaller than DMFRc. Defoliated leaves may nevertheless represent a significant source of infection, depending on the amount of infectious tissues. Mother lesions within the canopy generated more daughter lesions in the medium (or lower) layers of the canopy than at its upper level (DMFRc higher at the medium and lower layers of a canopy), whereas DMFRd values seemed to decrease with height in the canopy. A mechanistic simulation model that combines host growth and disease-induced defoliation was designed to simulate the respective contributions of the two components of the dual inoculum source of a diseased canopy (infected foliage and defoliated infectious tissues), and varying infectious periods in both sources. Simulations suggest that higher DMFRc values have a large polycyclic effect on epidemics whereas that of DMFRd is small, and that large effects of the infectious period of lesions in the canopy are found when DMFRc is high. Simulations using experimentally measured DMFRc and DMFRd values indicated much stronger epidemics in rainy compared to dry conditions for spore dispersal, but disease persistence in the latter. The implications of considering a dual source of inoculum in the course of a polycyclic process are discussed with respect to epidemic thresholds.

Novas fontes de resistência do feijoeiro comum à mancha angular

4conidios mL -1 . Os sintomas foram avaliados de 14 a 18 dias apos a inoculacao, utilizando uma escala de nove graus onde foram consideradas resistentes (reacao compativel) as plantas que apresentaram os graus de 1 a 3 e suscetiveis (reacao incompativel), as que apresentaram os graus de 4 a 9. Os genotipos ‘Ouro Negro’ e ‘LM 202202530’ apresentaram reacao de resistencia a oito e a sete dos oito patotipos utilizados, respectivamente. A cultivar BRS Requinte foi resistente a cinco patotipos e as cultivares BRS Pontal e Cornell 49-242 foram resistentes a apenas quatro patotipos. Palavras-chave adicionais: Phaeoisariopsis griseola, Phaseolus vulgaris, cultivares, patotipos.

Reaction of common bean cultivars to angular leaf spot in the greenhouse

Twenty-three common bean (Phaseolus vulgaris) genotypes were tested for reaction to angular leaf spot disease caused by Phaeoisariopsis griseola in the greenhouse. All genotypes were tested with monosporic isolates Ig 664 and Ig 669, characterized as pathotypes 63.23 and 63.19, respectively. Fourteen-day-old plants (vegetative stage V3) were inoculated with a suspension containing 2.5 × 104 conidia/mL. This suspension was applied to the upper and lower leaf surfaces. Disease symptoms were evaluated 14 and 18 days after inoculation by using a rating scale varying from 1 (no symptoms) to 9 (> 25% of leaf area with angular leaf spot lesions). Culivars BRS Valente, CNFC 10281, CNFP 10138, BRS Grafite, BRS Requinte, BRS Pontal, MAR 02, Cornell 49-242 and AND 277 showed resistance to both isolates. Cultivars Carioca Rubi, CNFC 9504, CNFC 10150 and Soberano were susceptible to both pathotypes. Pathotype 63.23 was the most virulent. The new resistance sources identified in this work could be used in common bean breeding programs aiming to incorporate resistance to angular leaf spot in new common bean cultivars.