Conventional Breeding of Taro (Colocasia esculenta) for Increased Resistance to Taro Leaf Blight

Taro (Colocasia esculenta) is a non-graminaceous monocot consumed primarily for its starchy corm.  A disease that threatens the sustainability of taro worldwide is Taro Leaf Blight (TLB) caused by the oomycete pathogen Phytophthora colocasiae.  This disease is a major problem of taro grown along the Hamakua Coast of Hawaii Island, where mean annual rainfall exceeds 3100 mm.  From 1994 through 2006, 123 taro cultivars from China, Hawaii, Palau, and Samoa were grown along the Hamakua Coast in plots containing five or 10 plants that were replicated over time.  Fresh weights of corms were measured after nine months.  During nine out of 12 years when TLB occurred, ratings of disease incidence were taken using the Horsfall-Barrett scale.  To determine the correlation between TLB resistance and yields, linear regression was conducted.  Increased blight ratings of cultivars (i.e. increased susceptibility to TLB) were correlated with significant decreases in fresh weight yields (r²= 0.18).  Multiple comparisons of the best were conducted on fresh weights of corms and blight ratings to determine the highest-yielding taro cultivars and the greatest resistance to TLB.  Cultivar Niumalu luau was found to have the highest fresh corm weight of 33,650 kg ha^-1 and the lowest TLB rating of 18.  Twelve other cultivars did not differ significantly from the best for fresh weights and of these, seven were Palauan cultivars.  All of these seven Palauan cultivars had TLB ratings that did not differ from the best.  Two commercial taro cultivars Bun Long and Maui Lehua had yields that were significantly lower than the best at 15,870 kg ha^-1 and 14,260 kg ha^-1, respectively.  Their TLB ratings were 33 and 30, respectively, indicating that conventional breeding of taro to improve TLB resistance also could improve yields.  Selected cultivars were hand-pollinated, seedlings evaluated, the most promising cultivars multiplied, and then field trials were conducted with plots of 30 plants replicated four times in a Randomized Complete Block Design.  In two trials during 2003-2005 that compared nine cultivars, cvs. Pauakea and Pa’akala (cross between Palauan cv. P-10 and Maui Lehua) had the highest fresh weight yields of 42,120 kg ha^-1 and 64,160 kg ha^-1, and TLB ratings of 23 and 16, respectively.  In a trial that compared 11 cultivars during 2006-2007, cv. MLxSushi-2 (cross between cvs. Maui Lehua and Sushi, a TLB-resistant cultivar) had the highest yield at 24,950 kg ha^-1 and a TLB rating of 18.