EVIDENCE FOR REDUCED SEXUAL REPRODUCTION OF ZYMOSEPTORIA TRITICI FOLLOWING TREATMENT WITH FLUXAPYROXAD AND IMPLICATIONS FOR INITIAL INFECTION OF WHEAT CROPS.

Zymoseptoria tritici (previously Mycosphaerella graminicola) is the causal agent of septoria tritici leaf blotch (STB), a globally important fungal disease of bread, feed and durum wheat. Airborne ascospores originating from over-winter crop residues are considered to be the primary source of initial infection. The active ingredient fluxapyroxad (BASF) belongs to the chemical group of carboxamides and is a succinate dehydrogenase inhibitor (SDHI) fungicide. Fluxapyroxad has strong efficacy against the key foliar diseases of winter wheat: STB, yellow rust and brown rust. The objective of the study was to test the effects of foliar applications of fluxapyroxad on the life cycle components of Z. tritici. We report here the effects of foliar applications of fluxapyroxad on initial inoculum and subsequent effects on disease level in the following crop. A large-plot field experiment was carried out in Herefordshire, UK, during 2010-2011. Fluxapyroxad was applied at conventional timings (Zadoks scale at z.32 and z.39, decimal growth stages GS32 and GS39) as a solo product and in co-formulation with epoxiconazole, and was evaluated by comparison with epoxiconazole and an untreated control for its effects on late season STB. Autumn ascospore release was measured in field plots using volumetric spore traps. A second wheat crop was established in the following year (2011-2012) using minimum tillage cultivation techniques, so that crop residues from the previous crop remained exposed. The second wheat crop was not treated with fungicide. The original plot boundaries from the preceding year were located and each plot was monitored for STB. A significant positive relationship was found between late season septoria leaf blotch severity and the mean number of ascospores released from over-winter crop residues. Foliar applications of fluxapyroxad reduced the number of ascospores released and this reduction correlated positively with pre-stem extension (z.31) STB severity in the following crop. A second field experiment was conducted in 2012 in which similar treatments were applied. After harvest crop residues (stems and leaves) were collected from the plots and stored over winter under ambient conditions. Ascospores were harvested onto agar plates following induced spore release in the laboratory. Reductions in ascospore release following foliar application of fluxapyroxad were confirmed. Furthermore, ascospores produced from crop residues from treated plots showed a reduction in length and rate of germination when compared with ascospores from untreated plots, which could indicate a potential fitness cost. This evidence indicates that applications of fluxapyroxad could reduce the risk of STB in following wheat crops in the same field. Applications to fields at a regional scale may reduce primary infections of newly emerging wheat crops over a wide area.