the Symposium on Wheats for More Tropical Environments - cimmyt

159Shinchunaga and descendants). severalmaterials have shown good resistanceover all localities during <strong>the</strong> fewdecades that <strong>the</strong>y have been tested (29).This fact has encouraged breeders andpathologists to attempt <strong>the</strong> difficult taskof breeding for resistance to a diseasecaused by a pathogen with a highvariability. high rate of multiplication.various means of dispersion andsurvival in <strong>the</strong> absence of <strong>the</strong> host.Breeding programs for scab resistanceare in progress now in China. Japanand Brazil. In order to select forresistance to scab. methodologies toassure <strong>the</strong> occurrence of <strong>the</strong> diseaseand to evaluate resistance are essential.Artificial InoculationInoculum productionSeveral culture media can be used toisolate and increase inoculum ofF. graminearum (7.72). Pieces of cornstalk with nodes. autoclaved with asmall amount of water (25). constitutean inexpensive media that produceshigh amounts of conidia and/orascospores. and allows several harvestsof inoculum; this method is now in usein Brazil. Cultures grown at 25°Cunder continuous flourescent light givegood production of conidia. Ascosporesare produced by some isolates keptunder natural conditions.Inoculation methodsSprays of a suspension of mycelia.conidia and/or ascospores are similarlyeffective in inducing disease whenenvironmental conditions are favorable(48.66.67.68.69). The concentration ofspores in successful inoculationsreported varies from 4 x ~03 to 8 x 106conidia/ml (1.14.55.56.72). In <strong>the</strong> testsconducted in Brazil. a spore suspensionwith 106 conidia/ml is applied. underboth field and greenhouse conditions.with a conventional sprayer.spread within <strong>the</strong> spike. As <strong>the</strong> mostdamaging infections occur betweenan<strong>the</strong>sis and <strong>the</strong> milk stage. I.e.. <strong>the</strong>period of highest susceptibility (1.2.8.10.11.12.34.54). <strong>the</strong> inoculation shouldbe made at flowering or a little later. InBrazil. mixtures of fifty or more isolatesfrom different localities are normallyused. in order to assure a pathogenicpopulation representative of <strong>the</strong> onethat occurs naturally in <strong>the</strong> field.Environmental conditionsIn greenhouses with temperaturesaround 25°C and relative humidityover 85% dUring <strong>the</strong> inoculation period(flowering to milk stage). <strong>the</strong>development of scab is very good. InPelotas. a marginal region for wheatproduction in Brazil. <strong>the</strong> warm. Windy.humid spring wea<strong>the</strong>r provides naturalconditions for a high incidence of <strong>the</strong>disease in <strong>the</strong> field almost every year.as can be seen in Table 1.Table 1. Natural occurrence of scab in wheatplots under field conditions, Pelotas,Brazil, 1979CultivarAburaInayamaNobeoka BozuNyu BayPekin 8ToropiPEL73007PEL73022PEL73157PEL73175FB5163JaralSuper XPV·lndusScabbed grains(0/0)19.652.073.986.3910.188.1823.2020.405.5116.2093.3381.8191.1795.28Injections of spore suspensions (300 to400 conidia per drop) in a central floretare very useful for testing resistance to