Characterizing the pathogenicity profiles of Phytophthora colocasiae

Abstract

The most significant plant disease affecting taro is taro leaf blight (TLB), caused by Phytophthora colocasiae. A taro breeding program was established at the University of Hawai‘i to develop taro varieties with improved characteristics including resistance to TLB. The program was initiated by crossing taro varieties with resistance to TLB from Palau, Indonesia, Guam, with a Hawaiian variety, ‘Maui Lehua’ taro, known to be susceptible to TLB. A previous study used detached leaf disc assays to challenge new hybrid cultivars with P. colocasiae isolate HPA 1, which was originally isolated from Panaʻewa, Hawaiʻi. Many of the hybrids were resistant to this isolate and were classified as resistant to TLB. However, when these cultivars were subsequently challenged with P. colocasiae isolate HPE 3 from Pepeekeo, Hawai‘i (approximately 10 miles from Panaʻewa), some cultivars that were resistant to HPA 1 were susceptible to HPE 3. Further, a number of cultivars that were susceptible to the HPA 1 isolates were resistant to HPE 3. This current study aimed to determine whether the P. colocasiae populations in Panaʻewa and Pepeekeo are homogenous with regard to pathogenicity. This was done using a panel of seven taro cultivars selected in the previous study in addition to cultivar ‘Bun-Long’ which was used as a susceptible control collected from Waiakea Research Station and a taro field in Pepeekeo. The cultivars were separated into four categories: cultivars that were resistant to both HPA 1 and HPE 3, cultivars that were susceptible to both HPA 1 and HPE 3, cultivars that were resistant to the HPA 1, but susceptible to HPE 1, and lastly, cultivars that were resistant to HPE 3, but susceptible to HPA 1. The cultivars were challenged against 20 isolates of TLB, 8 from Panaʻewa and 12 from Pepeekeo. These isolates were designated HPA 2 – 9 and HPE 4 – 15, respectively. The original solates HPA 1 and HPE 3 were also analyzed in this study to evaluate whether changes occured in these cultures over time. We used the pathogenicity patterns of these isolates to group them into pathotypes, which were determined using two trials of detached leaf disc assays. In addition, the influence of some environmental factors on the leaf disc assay were analyzed by comparing disc assays from leaves of the same variety from nursery plantings in Panaʻewa, with field plantings from Pepeekeo. The influence of leaf size on disc assays was also evaluated by comparing the results of disc assays on large (older) and small (younger) leaves from a single plant. The first trial showed that the Panaʻewa and Pepeekeo P. colocasiae populations consisted of multiple pathotypes resolved using the panel of eight selected cultivars. The second trial, performed ~ 2 weeks after the start of trial 1, revealed a change in pathogenicity patterns in a few P. colocasiae isolates. While some P. colocasiae isolates displayed a surprising degree of plasticity over time with regard to pathogenicity, the plants used in this study were unaffected by varying cultivation practices, location, and leaf site. Based on the results of this study, I conclude that P. colocasiae field populations are heterogenous, and that P. colocasiae cultures can exhibit changes in pathogenicity profiles within 2 weeks.