Effects of pyraclostrobin on leaf diseases, leaf physiology, yield and quality of durum wheat under Mediterranean conditions

A two-year field experiment was conducted to study the putative "health" effects of pyraclostrobin along with its fungicidal action on two durum wheat (Triticum turgidum L. subsp. durum) cultivars under rainfed, Mediterranean conditions. Five foliar treatments were applied: single applications [100 g active ingredient (ai) ha(-1)] at the end of tillering (BBCH 31, T-31) and the flag leaf stage (BBCH 39, T-39), application (100 g ai ha(-1)) at both growth stages (a total of 200 g ai ha(-1); T31+39), chemical control [T-CC; a tank mixture of myclobutanil (60 g ai ha(-1)) and fenpropimorph (750 g ai ha(-1)) at BBCH 31, 39 and 69 (end of flowering) growth stages] and untreated control (T-UC). The two predominant foliar diseases [powdery mildew (PM) caused by Blumeria graminis f. sp. tritici, and Septoria tritici blotch caused by Zymoseptoria tritici] were assessed five times, initiating at BBCH 30, and the area under disease progress curve (AUDPC) was calculated. At the end of flowering (BBCH 69), physiological traits were determined on flag leaf [nitrogen concentration (Leaf N), chlorophyll content as assessed by SPAD-502 (SPAD), specific leaf area (SLA)] and canopy [canopy temperature depression (CTD)]. Also, carbon and nitrogen isotopes were measured on flag leaf [carbon isotope discrimination (Delta(FL)), N-15 natural abundance (delta N-15(FL))] and mature grains (Delta(G) and delta N-15(G)) at BBCH 69 and 99 (harvest maturity), respectively. Pyraclostrobin applications suppressed both foliar diseases as AUDPC revealed. The T31+39 was the most effective treatment leading to highest grain and protein yields (GY and PY) while T-31 ranked second in yields indicating that early suppression of foliar diseases (T-31, T31+39) was the most beneficial. The negative correlations between yields and Delta(FL-G), a measure of flag leaf isotopic signature on grains, signified the importance of the prolongation of flag leaf photosynthesis as a result of foliar diseases suppression. On the other hand, foliar treatments had no significant effect on grain quality. The high-yielding cv. Elpida was tolerant to powdery mildew, water conservative during grain filling and more dependent on carbohydrates translocation to filling grains (higher Delta(FL-G)). The high-yielding growth season also showed higher grain protein concentration and vitreousness as a result of the lower diseases pressure, which was mirrored in higher flag leaf N, SLA and CTD. Concluding, the positive yield response of durum wheat to pyraclostrobin was not due to a "plant health" effect but owing to foliar diseases suppression, which allowed prolonged flag leaf photosynthesis.