the Symposium on Wheats for More Tropical Environments - cimmyt

202infection was obtained with lessfrequentfungicide applications. Spotblotch and head blight were scoredweekly. The average crop loss. ascalculated from all tests for <strong>the</strong> differentseasons. was 45% of <strong>the</strong> yield of <strong>the</strong>healthy plots; <strong>the</strong> maximum crop losswas 85% of <strong>the</strong> yield of healthy plots.For all practical considerations. thiswas a total crop loss. Severe foliarnecrosis and head blight was recordedin plots which were exposed to <strong>the</strong>natural epidemic. Yields of more than3 tlha were obtained in disease-freeplots. with a maximum yield of 3.6 tlharecorded. Such yields are an indicationof <strong>the</strong> potential for rainfed wheatproduction in Zambia if H. sattvum iscontrolled. ei<strong>the</strong>r genetically and/or byfungicide applications.Rainfed wheat production with frequentfungicide use is certainly out of <strong>the</strong>question. due to economicconsiderations. If <strong>the</strong> input of afungicide is going to be considered. itwould have to be a highly effectivechemical with very good persistence ata maximum of one or two applications.Fungicide tests were carried out with<strong>the</strong> Mexican wheat cultivars. Anorientation trial with <strong>the</strong> variety Sonora64 showed that plots which receivedtwo applications of various standardcontact fungicides did not have yieldsdifferent from those of <strong>the</strong> unsprayedplots. The objectives were changed and.instead of concentrating on <strong>the</strong> numberof applications. <strong>the</strong> emphasis wasplaced on finding an effective formula.Nine chemicals were evaluated on <strong>the</strong>variety Mexipak dUring <strong>the</strong> 1978season. Two spray intervals were used.with spraying starting from <strong>the</strong> firstsigns of visible infection, which was onemonth after seeding. Eight sprayings atweekly intervals were applied,compared to three sprayings atintervals of three weeks. Above-averagerainfall was recorded dUring <strong>the</strong> postfloweringperiod, prOViding <strong>the</strong> rightconditions for <strong>the</strong> development of <strong>the</strong>H. sattvum epidemic in <strong>the</strong> plots and,consequently, for proper screening of<strong>the</strong> chemicals. The fungicides whichwere tested were Captafol. Tridemorphmaneb.Triadimefon. Fentinacetatemaneb.Mancozeb. Metiram. Maneb.Benomyl and Methylthiophanate. Theunsprayed control plots yielded only0.2 tlha under high disease pressureconditions. Fentinacetate-maneb andTriadimefon were <strong>the</strong> most effectivechemicals in slOWing down <strong>the</strong>epidemic. Plots which were sprayedwith <strong>the</strong>se chemicals at intervals ofonce every three weeks yielded 0.8 tlha.Plots sprayed with Fentin at weeklyintervals yielded 1.7 tlha versus 1.0tlha for plots sprayed with Triadimefon;a significant reduction of foliar infectionand head blight was recorded for both.Head blight and node infection (stembreak) were better controlled byFentinacetate-maneb than byTriadimefon.These two chemicals were comparedduring <strong>the</strong> next season with three newproducts, RH2161, EL228 and ImazalU,and also with a tank mix ofFentinacetate-maneb with Tridemorphmaneb.Four sprayings with eachchemical were applied at intervals oftwo weeks, from flag leaf appearanceonwards. The high disease pressure of<strong>the</strong> previous season did not occur, and<strong>the</strong> hand-harvested control plots yielded1.9 tlha. Plots sprayed withTriadimefon. Fentinacetate-maneb and<strong>the</strong> tank mix yielded 3.4 tlha. 2.8 tlhaand 3.2 tlha, respectively; <strong>the</strong>se yieldswere not significantly different fromeach o<strong>the</strong>r. Plots which were sprayedwith RH2161 also yielded significantlybetter than <strong>the</strong> control plots. but <strong>the</strong>yields were lower than those of <strong>the</strong>Triadimefon plots; perhaps a higherdosage of RH2161 could have bridgedthis gap. EL228 and Imazalll did notcontrol <strong>the</strong> disease sufficiently. andplots which were sprayed with <strong>the</strong>sechemicals did not have yields differentfrom those of <strong>the</strong> unsprayed controlplots. The efficacy of Triadimefon andFentinacetate-maneb for <strong>the</strong> control of