A co-inoculation technique to rapidly screen citrus hybrids for resistance to both Alternaria Brown Spot caused by Alternaria alternata and Citrus Scab caused by Elsinoë fawcettii
Malcolm Wesley Smith, Debra Lorraine Gultzow, Toni Karen Newman, Sigrid Carola Parfitt & Andrew Keith Miles
A simple and inexpensive technique has been developed to allow the simultaneous screening of young hybrid seedlings for susceptibility to Alternaria Brown Spot (ABS) caused by Alternaria alternata and Citrus Scab (CS) caused by Elsinoë fawcettii. It is employed within six months of seed sowing, prior to hybrids being field planted, and has significantly improved breeding efficiency. By using the same culture techniques to multiply each pathogen, greater flexibility and ease of inoculation has been achieved. Plates of each pathogen are scraped/macerated to remove spores and other colony-forming units, strained through a coarse filter, combined and immediately sprayed onto vigorously growing seedlings. These seedlings are then incubated at ~25°C and high humidity for 4-7 days before being returned to the greenhouse. Hybrids susceptible to ABS develop symptoms within two weeks and are immediately discarded. A second round of culling is performed after four weeks when CS symptoms have expressed. The process is repeated three times to minimise ‘disease escapes’ and has virtually eliminated these diseases from our field plantings of new hybrids. Nucellar seedlings of genotypes known to be resistant and/or susceptible to each pathogen were used to confirm that mixing the two pathogens did not cause a change in symptom development for either disease.