RESISTANCE AND TOLERANCE TO HUANGLONGBING IN CITRUS

Huanglongbing (HLB) is severely impacting Florida citrus. Productivity declines in many HLB-affected genotypes are often accompanied with thinned canopies, and reduced fruit size and quality. Development of citrus cultivars resistant or tolerant to HLB is the best long-term control solution for this endemic disease in Florida. HLB was assessed in diverse cultivars in commercial groves with high HLB incidence. ‘Temple’ had the least HLB symptoms and lowest Liberibacter (Las) titer, while ‘Murcott’ and ‘Minneola’ had the most symptoms and highest titers. The USDA Ft. Pierce, FL farm is managed to reveal genotype responses to HLB. Some current cultivars and hybrid seedlings demonstrate HLB resistance/tolerance, at least to the strain(s) of Las present in this location. Citrus trifoliate is the best documented citrus resistance source, having low Las titer even when grafted onto severely-infected rootstocks. Some cultivars and hybrids have abundant foliage symptoms, but full canopies and seemingly normal fruit set and size. In three years of a replicated trial of ‘Triumph’(T), ‘Jackson’(J), ‘Flame’(F), and ‘Marsh’(M) grapefruits or hybrids, HLB symptoms were severe in all trees and Las titers were similar. F and M were almost completely defoliated in some years while T and J had full canopies. Cumulative fruit per tree was greater for T and J (255, 220) than for F and M (29, 66). T and J fruit met commercial standards and was normal size but the fruit of F and M was unacceptable. Evidence is mounting that useful resistance/tolerance to HLB is present in cultivated citrus and sexually compatible hybrids.