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  • Author or Editor: Charles E. McCulloch x
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Susan C. Miyasaka, Charles E. McCulloch and Scot C. Nelson

Taro leaf blight (TLB), caused by the oomycete pathogen Phytophthora colocasiae, is a worldwide disease that threatens the sustainable cultivation of the tropical root crop taro (Colocasia esculenta). To evaluate taro germplasm from Asia, Hawai‘i, and several South Pacific Islands for resistance to TLB, 119 cultivars were planted along the Hamakua Coast of Hawai‘i (mean annual rainfall of 130 inches) in plots containing five or 10 plants that were replicated over time from 1993 through 2005. Fresh and dry weights of corms were measured after about nine months, with rotten portions removed and weighed. When epidemics of TLB occurred (in nine out of 12 years), visual estimates of disease severity on leaves were assessed using a modified Horsfall–Barratt scale. The correlations between mean dry weight yields for each cultivar and mean severity of TLB, and, respectively, between mean yields and mean severity of corm rots were calculated. As severity of TLB or severity of corm rots increased (suggesting increased susceptibility of particular cultivars to TLB or corm rots), mean dry weight yields decreased significantly (r 2 = 0.37 and 0.22, respectively). “Multiple comparisons with the best” (MCB) were conducted on fresh and dry weight yields, severity of TLB, severity of corm rots, percentage dry matter of corm, and consumer acceptance. Five cultivars were found to be “among the best” with: 1) fresh or dry weight yields that did not differ from the highest level; 2) severity ratings for TLB that were significantly lower than the highest level, suggesting TLB resistance; and 3) percentage of corm rots that were lower than the highest level, suggesting disease resistance. These cultivars, four of which originated from Palau, were Dirratengadik, Merii, Ngesuas, Ochelochel, and Sawa Bastora. Two commercial cultivars from Hawai‘i, Bun Long and Maui Lehua, had fresh and dry weight yields that were significantly lower than the maximum and severity of TLB injury that did not differ from the highest level, indicating that conventional breeding of taro to improve TLB resistance could improve yields of commercial taro cultivars, particularly in areas where epidemics of TLB occur.

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Susan C. Miyasaka, Charles E. McCulloch, Graham E. Fogg and James R. Hollyer

Taro (Colocasia esculenta L. Schott) is a root crop widely grown in the Tropics. To determine the optimum plot size for taro field trials, fresh and dry weights of individual corms were collected from two field trials conducted under flooded culture and two conducted under upland culture. For a given maximum test plot with a single border row surrounding inner measured plants, all possible combinations of smaller plot sizes were investigated. A plot size was defined as a given number of adjacent plants. A strong linear relationship was found between the natural logarithm of variance of yield and the natural logarithm of plot size. Expressed on the non-log-transformed scale, the point of maximum curvature in this relationship indicates a sudden decrease in advantage to larger plot sizes and is taken as optimum. Calculating maximum curvature mathematically, optimum plot size was 21 inner plants (5.7 m2) for the second flooded trial and 18 inner plants (4.9 m2) for the second upland trial. Another method of estimating optimum plot size minimized the cost per unit of research data by using the index of degree of correlation between neighboring plots. In three of four trials, the optimum plot size ranged from 16 to 24 inner plants (4.3 to 6.5 m2). In this second method, we calculated a non-linear relationship between plot size and outer border plants to estimate the fixed and per-unit cost of a single border row surrounding the inner measured plants. Both methods of calculating optimal plot size sometimes resulted in estimates that exceeded the maximum test plot size for particular field trials, indicating limitations of each method and the importance of managing field trials to ensure uniformity across treatments. No evidence of spatial autocorrelation was found in the corm yield of taro, indicating that the two methods used were adequate in calculating optimum plot size. In addition, we conducted an analysis based on statistical power but found that plot size did not materially affect the power to detect differences between treatments. To our knowledge, this is the first report of optimum plot size for field trials of taro.

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Susan C. Miyasaka, Jeff B. Million, Nguyen V. Hue and Charles E. McCulloch

Possible boron (B) deficiency symptoms were observed on avocado (Persea americana Mill. `Sharwil') grown in Kona, Hawaii. To determine the B requirement of young, `Sharwil' avocado trees, two greenhouse experiments were conducted. In a soil study, seven B treatments (0, 3.7, 11, 22, 44, 89, and 178 mg·kg–1 soil fines) were applied to 1-year-old grafted `Sharwil' avocado trees grown for 13 weeks in a Tropofolist soil. Due to the low and variable fractions of soil fines in this rocky soil, extractable soil B concentration did not appear to be a good predictor of B requirements by avocados. Adequate foliar B concentrations in `Sharwil' avocado trees based on dry weight and area of new leaves ranged from 37 (±3) to 65 (±4) and from 31 (±10) to 78 (±13) mg·kg–1 (dry-weight basis), respectively. (Means are followed by standard errors of the mean in parentheses.) In a hydroponics study, 6-month-old grafted `Sharwil' avocado trees were supplied with four levels of B (0, 1, 10, and 100 μm). At 11 months after B treatment initiation, leaves with deformed margins and a “shot-hole” appearance were first observed at a solution level of 0 μm B. At 14 months after B treatment initiation, foliar B concentrations that were associated with 12% to 14% incidence of deformed leaves ranged from 9.8 to 13.5 mg·kg–1 (dry-weight basis). Although `Sharwil' avocados are reportedly susceptible to B deficiency, foliar B concentrations required for adequate growth and those associated with B deficiency symptoms are similar to those for other cultivars.