Fitness of Cercospora beticola field isolates resistant and sensitive to demethylation inhibitor fungicides

Isolates of Cercospora beticola resistant to fungicides that inhibit sterol demethylation (DMIs) were collected from sugar beet fields in Northern Gre ece. Fitness of these isolates was compared to that of DMI-sensitive isolat es. The parameters measured were competitive ability both under growth cham ber and field conditions, mycelial growth, spore germination, germ tube len gth, incubation period, virulence and spore production. The competitive abi lity under growth chamber conditions was measured for 4 pairs of one resist ant and one sensitive isolate. Results showed that after 4 disease cycles, in 2 out of 4 pairs tested, the resistant isolates competed well with the s ensitive isolates, but in the remaining two pairs the frequency of the resi stant isolate decreased significantly. The competition experiment in the fi eld was carried out by inoculating field plots with a conidial suspension c onsisting of a spore mixture from all the resistant and all the sensitive i solates used in this study. Results showed that at the end of the growing p eriod the frequency of the resistant isolates had slightly decreased (P < 0 .05). The measurements of fitness components of individual isolates showed that the resistant isolates had significantly lower (P < 0.05) virulence an d spore production than the sensitive isolates, while no significant differ ences (P > 0.05) to the remaining 4 fitness components, were detected. With correlation analysis it was determined whether there is a relationship bet ween values of each fitness component and the level of sensitivity to flutr iafol of individual isolates. The correlation coefficients for virulence (r = 0.45) and spore production (r = 0.41) were significantly different from 0 (P < 0.05), indicating that resistance to DMIs affected, to some degree, the fitness of the resistant isolates.