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

doi:10.4322/crt.icc043

Abstract

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 Elsinoe 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.

Highlights

Alternaria Brown Spot (ABS) and Citrus Scab (CS) are long-standing disease problems in many citrus producing countries and yet solutions via genetic resistance have been poorly explored
The nature of Elsinoë spp. associations with Citrus spp. is diverse and confusing (Miles et al, 2015b) it is generally accepted that two species (E. fawcettii and E. australis) are responsible for commercially important disease impacts (Timmer, 2000)
Surveys of field progeny blocks showed an extremely low incidence of hybrids with ABS symptoms (

Summary

Introduction

Alternaria Brown Spot (ABS) (caused by a pathotype of Alternaria alternata) and Citrus Scab (CS) (caused by pathotypes of Elsinoë fawcettii) are long-standing disease problems in many citrus producing countries and yet solutions via genetic resistance have been poorly explored. The nature of Elsinoë spp. associations with Citrus spp. is diverse and confusing (Miles et al, 2015b) it is generally accepted that two species (E. fawcettii and E. australis) are responsible for commercially important disease impacts (Timmer, 2000). Each of these species has several pathotypes which can be distinguished on the basis of their host range, and they have variable geographical distribution. E. australis pathotypes, such as those causing Sweet Orange Scab, long established in South America (Bitancourt & Jenkins, 1937) have still to become established in many other citrus producing countries, including Australia. It should not be implied that this technique would work for pathotypes of E. australis, such as those causing Sweet Orange Scab

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Conclusion