Screening of citrus germplasm to identify tolerant and resistant genotypes to Plenodomus tracheiphilus (Petri) Gruyter, Aveskamp & Verkley

“Mal secco” is one of the most harmful disease of citrus and it is widespread in different Mediterranean countries. It is a tracheomycotic disease caused by the fungus Plenodomus tracheiphilus (Petri) Gruyter, Aveskamp & Verkley. Besides lemon (Citrus limon L. Burm.) “mal secco” pathogen infects, with high yield losses, other citrus species, including citron (C. medica L.), lime (C. aurantifolia Christ.), bergamot (C. bergamia Risso), sour orange (C. aurantium L.) and volkamer lemon (C. volkameriana Ten. Et Pasq.). With the aim of identifying sources of tolerance to “mal secco”, we started a phenotypic survey in the CREA lemon and lemon-like germplasm located in Acireale, Italy. The germplasm was planted in 2002 and included 25 lemon clonal selections and autotetraploids, as well as 17 citron and lemon hybrids. Other citrus species reported as tolerant or resistant were also included in the survey as reference. All genotypes were replicated twice or three times and were planted in the same field block with high pressure of P. tracheiphilus. A few very susceptible lemon clones died within 3 to 5 years after planting, and some others declined slowly without a significant ability to recover from the disease. Phenotyping for “mal secco” symptoms started in 2018 and was performed three times on the remaining clones. A molecular screening by PCR was also performed to detect P. tracheiphilus in old flushes, new flushes and branches. The results revealed important sources of tolerance in the germplasm. The pathogen was detected by PCR in all lemon clones, many lemon hybrids and citron hybrids, also in plants without clear symptoms, suggesting the ability of some genotypes to recover from the disease. Interestingly, an autotetraploid lemon seems to be immune to the disease under natural pressure, since P. tracheiphilus was not diagnosed by PCR and visual screening. The survey was also helpful to identify the most suitable parents for the development of new breeding populations.