the Symposium on Wheats for More Tropical Environments - cimmyt

193temperatures. but this difference Isvastly reduced in <strong>the</strong> humid tropics.Differences in resistance in <strong>the</strong> breadwheats are perceivable. but are smallunder such conditions (4.14). Thepossible use and value of this differenceneeds fur<strong>the</strong>r evaluation. It Is possiblethat <strong>the</strong> resistance in barley toH. saltvum may also disappear in <strong>the</strong>humid tropics. but this subject has notreceived sufficient attention. Triticaleseems to have a greater degree ofresistance. but it has not been tested asextensively as <strong>the</strong> bread wheats. Thedurum wheats appear not to be welladapted. and so have received littleattention in <strong>the</strong> tropical environments.These differences recorded in resistanceto <strong>the</strong> foliar phase of <strong>the</strong> disease mayormay not apply to <strong>the</strong> seedling blightand root rot phase. No comparativework has been done on this subject in<strong>the</strong>se environments. but <strong>the</strong>re is someevidence that tolerance and degrees ofresistance to <strong>the</strong> root-attacking phase isoperative from temperate environments(26).The inoculum potential ofH. sattvumin tropical environments is severe. andit is found in abundance on numerousgrass species (4.8.14.17). The role ofinoculum potential in overcomingresistance in temperate climates hasbeen described (26) and. obviously. alsowill be an important factor in tropicalenvironments.Information on <strong>the</strong> possible resistanceto fusarium root rots Is limited (26). and<strong>the</strong>re is none currently ava1lable fortropical environments. The prospectsfor resistant or tolerant varietiesappears to be limited.Agronomic practicesThere are no established culturalpractices that can be recommendedwith confidence at this time for limitingseedling blights or root rots in tropicalenvironments. A number of possibi11tiesexist. but <strong>the</strong>y require carefulevaluation. For example. S. rolfstt isfavored in saturated soU-moistureconditions. Even small differences indrainage and root aeration appear tohave major effects on <strong>the</strong> growth andhealth of wheat and barley. There is aquestion of whe<strong>the</strong>r sowing on ridgesmight be beneficial to reducing rootrots. It has been observed that Wide-rowspacing has an effect on <strong>the</strong>development of <strong>the</strong> follar phase ofH. sattvum. but it is not known if thishas a carry-over effect on <strong>the</strong> root rotphase. The implications of weedcontrol. soU tlllage. ridging. sowingdepth. date of sowing. irrigationpractices. rotation and no-tlllage havenot been critically evaluated. althoughfield observations suggest <strong>the</strong>y may beextremely important in certain rootdisease situations (6.7.11.23).Chemical controlLimited information Is ava1labl~ on <strong>the</strong>effectiveness of chemical treatments onroot rots and seedling blights in moretropical environments. An initialevaluation on fungicide treatment madein 1984 at Central Luzon stateUniversity in <strong>the</strong> Philippines was oflimited success. Seed treatmentreduced infections by 53% over <strong>the</strong>control plots. but <strong>the</strong> level of infectionwas stlll high in <strong>the</strong> treated plots (27).The use of systemic fungicides or oneswith a greater fungal specificity mayprovide better control (2.22).A seed treatment combined with one.or possibly two. foliar applications. toprovide control of soU and seed-bornepathogens and to reduce <strong>the</strong> foliardisease-infection phase. appears to havemerit (4.16). Its combination withagronomiC practices to reducepredisposition of wheat and barley toroot diseases could fur<strong>the</strong>r enhance <strong>the</strong>effectiveness of chemical seed and foliartreatments.