Causal agent(s) and transmission
Candidatus Liberibacter solanacearum is not cultivable bacteria (obligatory plant parasite) which is limited to the plants phloem.
Polymorphism on the genome of the bacterium resulted in the description of five haplotypes (or types of strains). Haplotypes A and B are related to Solanaceae and are transmitted by psylla Bactericera cockerelli whereas the haplotypes C, D and E are described on Apiaceae and are transmitted by the psylla Bactericera trigonica and Trioza apicalis.
Zebra Chip disease has been described for the first time in Mexico in 1994. Since then, it has spread to several countries in Central America and in North America (it is reported in 15 States in the United States) as well as in New Zealand and Norfolk Island.
This disease has never been reported in Europe in crops of potato or other solanaceous plants; haplotypes A and B of Ca. L. solanacearum and their vector Bactericera cockerelli have never been described in Europe. The strains which have been described on Solanaceae (A and B) as well as their vector psylla, Bactericera cockerelli, are on the A1 list of EPPO (absent quarantine organisms of the EPPO region). They are also listed in France as a category I pest by Ministerial Decree of December 15, 2014.
The haplotypes C, D and E have been described on Apiaceae in several European countries on carrot and celery and their (psylla) vectors are not regulated in Europe.
Symptoms on foliage
During crop season, infected plants are stunted and have an erected port. The leaves, sometimes curled, show chlorotic areas with some mottling and a purplish color, especially at the level of the apical area (photo 1). The stems may have “zig zag” patterns, and with short and thickened nodes and aerial tubers. Heavily infected plants have foliage that looks burnt (photo 2)
Symptoms on tubers
Tubers of infected plants are often small, many and deformed. Germination is affected with fail and thin sprouts. The point of attachment of the stolon is marked and pinkish. The symptoms observed on cut tubers are a browning of the vascular ring associated with necrotic spots and streaks in the tuber flesh (photo 3). The symptoms are more pronounced after frying and gave their name to the disease (Zebra chip, photo 4).
The strains of Ca. L. solanacearum which have been described on Solanaceae (A and B) and are responsible for Zebra chip disease on potato and their vector psylla, Bactericera cockerellii (photo 1) are on the A1 list of EPPO (quarantine organisms, which are absent of the EPPO region).
In Europe and healthy areas, prevention of the introduction of the disease is based upon the phytosanitary regulations and preventive measures to avoid the introduction of the Solanaceae haplotypes and the vectors psylla.
In the countries where the disease is present on Solanaceae (United States, Central America), insecticides treatments against the vectors (psylla) can reduce the risk of contamination.