Screening of Oil Palm Varieties Resistant against the Leaf Spot Disease Caused by Curvularia oryzae Bugnic

Abstract

Curvularia leaf spot (CLS) is one of the diseases caused by Curvularia oryzae that can devastatingly damage both quality and quantity of oil palm seedlings in Thailand. Chemical fungicides have been intensively applied to control transmission of this disease. However, pathogen gains resistance to fungicide and causes low effectively. An alternative or complementary cost-effective and environmentally friendly approach is to find resistant varieties for disease management. In this study, a total of 122 lines of Dura female plant, 2 lines of Pisifera male plant, and 4 Tenera commercial varieties (A, B, C, and SUP-PSUI) were screened by Curvularia inoculation using detached leaf method. Two weeks after inoculation, the results showed 13 Dura lines were highly resistant to CLS (0% disease incidence), whereas line 129 and the commercial variety B were highly susceptible (100% disease incidence). From 13 Dura highly resistant lines, nine Tenera hybrid lines were selected to test against CLS under greenhouse conditions by pathogenicity test. Among nine Tenera hybrid lines, disease symptom was found to delay up to 12 days after inoculation in line 187. The disease score was 0.25 and disease incidence was 5% at 20 days after inoculation. Enzyme activity at 48 h of chitinase and B-1,3-glucanase after inoculations were 17.84 = 1.46 and 14.23 = 1.31 U mL"', respectively, higher than that of susceptible lines. CLS disease was found at 3 days after inoculation at the susceptible line 129. The study found that disease score was 4.55, disease incidence was 91% at 20 days after inoculation, and the lowest PR-proteins at 48 h after inoculation at 3.27 +0.27 and 2.05 +0.70 U m . To classify hybrid crosses as susceptible and resistant variety, the hybrid line 129 and line 187 were selected to test chitinase and B-1,3-glucanase activities every 24 h from 0 to 168 h. The results showed enzyme assay of line 187 could express both of PR-proteins higher than line 129 from 24 to 168 h. At 48 h after inoculation, line 187 could express the highest chitinase and B-1,3glucanase activities at 14.03 +0.87 and 13.51 + 1.04 U mL while line 129 could express chitinase and B-1,3-glucanase activities at 3.76 + 0.41 and 4.31 +0.83 U mL-'. The SDSPAGE showed accumulation of 22, 25, and 33 kDa, chitinase and B-1,3-glucanase proteins in inoculated line 187, whereas no chitinase and B-1,3-glucanase proteins were observed in line 129. In addition, 3 oil palm Tenera hybrid genotypes (line 138, line 187, and line 203) were tested for field conditions at Nuea Khlong district, Krabi province and Yan Ta Khao district, Trang province from February 2016 to October 2016. The results showed the selected Tenera hybrid genotypes had more resistance than commercial varieties especially during a raining season in July (2nd data recorded) that was severe CLS disease in oil palm nursery. This study suggested that 3 oil palm Tenera hybrid genotypes line 138, line 187, and line 203 were candidates to be a database and useful for breeding and developing new oil palm variety resistant to CLS disease.