Effects of Temperature and Moisture on Conidia Germination, Infection, and Acervulus Formation of the Apple Marssonina Leaf Blotch Pathogen (Diplocarpon mali) in China.

Apple Marssonina leaf blotch (AMLB; Diplocarpon mali) is a severe disease of apple that mainly causes premature leaf defoliation in many apple growing areas worldwide. AMLB epidemic development is closely related to temperature and rainfall. In this study, the effects of temperature and moisture on conidium germination, infection on leaves, and acervulus production were investigated under controlled environments. The temperature required for conidium germination and infection ranged from 5 to 30°C, with the optimum at approximately 23°C. The temperature required for acervulus formation was slightly higher, with the optimum at 24.6°C. Wetness was needed in order for conidia to germinate and infect; only a few conidia germinated at 100% RH. However, lesions can produce acervuli in dry conditions. The minimum duration of leaf wetness required for conidia to complete the entire infection process was 14, 8, 4, and 6 h at 10, 15, 20, and 25°C, respectively. A model describing the effect of temperature and leaf wetness duration was built. The model estimated that the optimum temperature for conidial infection was 22.6°C and the minimum wetness duration required was 4.8 h. This model can be used to forecast D. mali conidial infection to assist in disease management in commercial apple production.

A novel wall-associated receptor-like protein kinase gene, OsWAK1, plays important roles in rice blast disease resistance.

Wall-associated protein kinases (WAKs) are a new group of receptor-like kinases (RLK) recently identified in Arabidopsis. A cDNA encoding a novel WAK was isolated from rice and was named OsWAK1 (Oryza sativa WAK). The deduced amino acid sequence of OsWAK1 showed 27.6% identity to WAK2 from Arabidopsis. OsWAK1 not only has the ability of autophosphorylation but also can phosphorylate OsRFP1, a putative transcription regulator recently identified in rice. OsRFP1 strongly interacts with the kinase domain of OsWAK1. This demonstrated that OsWAK1 is a functional protein kinase. A fusion protein of OsWAK1 with GFP was found to be localized on the cell surface. Plasmolysis experiments further revealed OsWAK1 is associated with the cell wall. Northern blotting analysis showed that infection of the rice blast fungus, Magnaporthe oryzae significantly induced the OsWAK1 transcripts, and the accumulation of OsWAK1 mRNA occurred earlier and was more abundant in rice leaves infected with an incompatible race than with a compatible race of the blast fungus. OsWAK1 was also induced after treatment by mechanical wounding, SA and MeJA, but not by ABA. These results imply that OsWAK1 is a novel gene involved in plant defense. Furthermore, six transgenic rice lines with constitutive expression of OsWAK1 became resistant to the compatible race. However, OsWAK1 expression was undetectable in leaves, stems and flowers but very weak in roots under normal growth conditions. This provides functional evidence that induction of OsWAK1 as a novel RLK plays important roles in plant disease resistance.