the Symposium on Wheats for More Tropical Environments - cimmyt

196Chemical Control Measuresfor <strong>the</strong> Major Diseases of Wheat, withSpecial Attention to Spot BlotchY.R. Mehta and S. Igarashi, Instituto Agronomico de Parana,Londrina, Parana, BrazilAbstractSpot blotch. rusts, and scab are some of <strong>the</strong> most important diseases in manytropical countries. Frequently, more than one disease appears at <strong>the</strong> same time.Success in chemical control of a disease complex depends on <strong>the</strong> use ofanappropriate spraying schedule which takes into consideration 1) appropriatefungicide and dosagefor <strong>the</strong> diseases and <strong>the</strong> variety in question. 2) time offirstspraying. 3) number ofsprayings and 4) <strong>the</strong> interval between sprayings. For adisease complex. Maneb or Mancozeb should always be used in afungicidalmixture. Spot blotch is well controlled by three tofour applications ofManeb orMancozeb or three applications ofPropiconazole.Although yields by some farmers intropical areas, and more particularly inLatin American countries, exceed 3,000kg per hectare, <strong>the</strong> average yieldsduring <strong>the</strong> past ten years have variedbetween 1,000 and 1.200 kg/ha. Theprimary causes of such low yields areprobably adverse climatic conditionsand severe disease epidemics.One of <strong>the</strong> major problems for wheatproduction in most tropical countries is<strong>the</strong> severe disease complex. Such adisease complex may include majordiseases like leaf rust (Pucciniarecondita Rob. ex Desm.) spot blotch(Bipolaris sorokiniana Sacco ex.Sorokin, syn. Helminthosporiumsativum P. K. and 8.). and head blightor scab (Fusarium graminearum)Schwabe (Gibberella zeae (Schw.)Petch. These diseases can be controlledby <strong>the</strong> use of resistant varieties;never<strong>the</strong>less. until resistant varietiesare available. chemical control shouldreceive due priority.At present, <strong>the</strong>re are more than 20different fungicides available for <strong>the</strong>major wheat diseases. and new ones arecontantly being introduced into <strong>the</strong>market. When considering fungicideuse. economic aspects must be takeninto consideration. The success ofchemical control depends mainly on anappropriate spraying schedule. andconsideration must be given to1) appropriate fungicide and dosage for<strong>the</strong> disease ana <strong>the</strong> variety in question.2) time of first spraying. 3) number ofsprayings and 4) <strong>the</strong> interval betweensprayings.An appropriate spraying schedule hasbeen established for Brazil andParaguay. and it is being used withgood success; <strong>the</strong> schedule needs to befur<strong>the</strong>r established on a regional basis.However. considering <strong>the</strong> commondisease complex problem among LatinAmerican countries, <strong>the</strong> existingspraying schedule can be used in o<strong>the</strong>rcountries with some modifications, ifnecessary.Fungicides and DosageFungicides are normally selected andrecommendf'd as a result of yielddifferences between treated plots andcheck plots. During <strong>the</strong> past severalyears. more than 30 fungicides withdifferent active ingredients have beentested on <strong>the</strong> most popular varieties inBrazil. ei<strong>the</strong>r alone or in combination.