Quantification of Pseudocercospora fuligena in Tomato Lines Carrying Introgressions from Solanum habrochaites Using a qPCR Assay.

Black leaf mold (BLM), caused by Pseudocercospora fuligena, is a major plant growth- and yield-limiting factor for tomato production in the humid tropics. A library of 90 introgession lines (ILs), the BLM-resistant donor Solanum habrochaites, and the BLM-susceptible recurrent S. lycopersicum parent (RP) were visually phenotyped under natural infection conditions in a nethouse in central Thailand. ILs showing no and severe BLM symptoms were among the phenotypes. Because visually classifying different phenotypes of BLM symptoms can lead to erroneous results due to the similarity of BLM symptoms to other abiotic and biotic problems, a SYBR Green-based quantitative polymerase chain reaction (qPCR) assay for fungal DNA was developed. Correlation of the results from visual phenotyping of plants grown in Thailand under natural infection conditions with qPCR test-based quantities of the tomato leaf fungal DNA content, while significant, explained only 39% of the relationship across the library population. Based on this phenotyping in Thailand, selected ILs were artificially infected in a growth chamber in Germany with a P. fuligena isolate from Thailand. The results of the natural infection could be verified for the selected ILs in the growth-chamber experiment. A close correlation (R = 0.87) existed between the visual disease rating and fungal DNA content in leaves in the latter experiment. Three lines without visual symptoms and very low amounts of P. fuligena-specific rDNA shared an overlapping introgression on chromosome one that may be associated with BLM resistance.