Evaluation of key meteorological determinants of wintering and flowering patterns of five rubber clones in Xishuangbanna, Yunnan, China.

All rubber tree clones (Hevea brasiliensis) exhibit regular annual wintering characterized by senescence and abscission of leaves. After 3-4 weeks, this is followed by the onset of new leaves. It is likely that the timing of leaf onset affects the susceptibility of rubber trees to rubber powdery mildew disease, as this predominantly infests young leaves. However, little information is available on the phenological behavior of different rubber clones, or how meteorological factors affect such behavior. We assessed the wintering and flowering patterns of five rubber clones in Xishuangbanna, southwest China, based on observations made from 1978 to 2011, and evaluated how these patterns responded to different meteorological factors. Partial least squares regression was used to analyze the timing of defoliation, refoliation, and flowering. Our results showed that the two clones RRIM 600 and GT1 defoliated during the last week of December and refoliated in the last week of January, and clones Yunyan 277-5, Yunyan 34-4, and PR 107 defoliated during the first week of January and refoliated in the second week of February. The number of hours of sunshine during both the rainy season and the cold dry period in the dry season were important determinants of phenological changes in the rubber trees. Similarly, higher temperatures tended to delay the onset of defoliation and refoliation, and were a triggering factor for the onset of flowering. These results may help rubber cultivators to schedule appropriate disease control measures, as well as to design hybridization programs aiming at the production of clones which are resistant to foliar disease.

Taxonomic revision and phylogenetic analyses of rubber powdery mildew fungi.

Powdery mildew is a fungal disease that infects a wide range of plants, including rubber trees, which results in a reduction of latex yields of up to 45%. The causal agent of powdery mildew of rubber was first described as Oidium heveae, but later morpho-molecular research suggested that in the past, O. heveae has been confused with Erysiphe quercicola. However, it is still under debate whether the causal agent should be classified as a species of the genus Erysiphe emend. or Golovinomyces and Podosphaera, respectively. Therefore, the aim of this study was to undertake the morpho-molecular characterization of powdery mildew species associated with rubber trees, thus resolving these taxonomic issues. Morphological observation under light and scanning electron microscopes (SEM) clearly identified two morphotypes of the rubber powdery mildew. With the support of morphological and phylogenetic data, one of the two morphotypes was identified as the asexual morph of E. quercicola, while the second morphotype is still insufficiently known and according to the morphological results obtained we assume that it might belong to the genus Golovinomyces. More collections and additional molecular data are required for final conclusions regarding the exact taxonomic position of the second morphotype of rubber powdery mildew and its relation to the name O. heveae. The haplotype analysis identified eight haplotype groups of E. quercicola indicating the high genetic diversity of the species.