Differential Response of Taro (Colocasia esculenta) Cultivars to Taro Leaf Blight

Taro (Colocasia esculenta) is a non-graminaceous monocot consumed primarily for its starchy corm.  It is a major staple crop in the Pacific, and is grown widely in the Caribbean, Africa, and Asia.  A major disease that threatens the sustainability of taro is Taro Leaf Blight (TLB) caused by the oomycete pathogen Phytophthora colocasiae.  Two methods were used to determine TLB resistance within the taro germplasm: a) field evaluation at five months after planting based on naturally-occurring epidemics of TLB; and b) excised leaf assay that challenges leaf disks with zoospores of P. colocasiae. Using both methods, resistance to TLB has been found within the taro germplasm.  We hand-pollinated two taro cultivars that appeared to be TLB-resistant based on the field assay.  Then, we challenged 76 of the resulting progeny using the excised leaf assay with zoospores of two strains of TLB that were isolated from the Island of Hawaii.  Interestingly, individual progeny responded differentially to two strains of P. colocasiae (HPA1 and HPE1), with some resistant to both strains, some resistant to strain HPA1 only, some resistant to strain HPE1 only, and some susceptible to both strains.  Correlation between TLB resistance to each strain was positive and significant (P = 0.001); however there was no significant correlation between normalized TLB resistance in either excised leaf assay and that based on field evaluation of these same progeny.  Further studies are being conducted to determine whether: 1) additional strains of P. colocasiae are present at the site where the field evaluation was conducted; 2) numbers of zoospores differ between the laboratory assay and the field; or 3) there are other critical factors involved in field-based TLB resistance (e.g., orientation of leaf blades) that are not assayed under laboratory conditions.