PN-5224 



Figure 9. — A plant of D. rotundata with mosaic disease in extreme form. 



amount of damage to the seed pieces is tolerated in exchange for 

 good control of weeds. Several preemergence herbicides have been 

 recommended. In Trinidad atrazine^ has been used at 3 kilograms 

 per hectare. Ametrjhi* at 3 to 4 kilograms per hectare has also 

 given good results. In any particular locality, herbicide use should 

 be tested before recommendations are made. 



After the effects of preemergence herbicides have worn off, 

 weed control is usually achieved by spot application of a contact 

 herbicide or by hoeing. The roots of yams are very shallow, and 

 cultivation should be done with care. Once the vine is well estab- 

 lished, it may shade most weeds sufficiently to prevent their 

 growth. 



Insects and diseases 



Although D. rotundata and D. cayenensis are fairly free of 

 pests and diseases, a few serious conditions do occur. These may 

 drastically reduce yields. 



The most serious of these diseases produces symptoms that 

 suggest a viral agent. Called the green-banding virus (26), or 

 mosaic disease in Puerto Rico and shoestring disease in Africa, 

 the disease produces symptoms of mosaic — blotching, crinkling, 

 distortion of leaves, and in its extreme phase, reduction of the 

 leaf to a thin twisted sliver (fig. 9). As symptoms become severe. 



3 2-Chloro-4- (ethylamino) -6- (isopropylamino) -s-triazine. 



^ 2- ( Ethylamino) -4- (isopropylamino) -6- (methylthio) -s-triazine. 



24 



