Zebra Chip Disease Development in Relation to Plant Age and Time of 'Candidatus Liberibacter solanacearum' Infection.
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A 2-year field study was conducted to evaluate plant susceptibility to 'Candidatus Liberibacter solanacearum', the putative causal agent of zebra chip disease (ZC). Incubation period of ZC, the rate of symptom progress, and the rate of pathogen population growth were evaluated for individual plants infested on different weeks after emergence. In foliage, incubation period was between 21 and 28 days. The pathogen was detected within leaf tissue in 3 to 4 weeks, regardless of the time of infestation. The rates of foliar symptom progress and pathogen population growth were uniform among all infestations. Although symptoms were observed in only 1.3% of tubers from plants infested 2 weeks before harvest, 74% of these tubers tested positive for the pathogen. There was a positive correlation between symptom severity and titer in the foliage. Within tubers, however, the relationship was negative but nonsignificant. Pathogen titer reached detectable levels some time between 7 to 14 days following infestation. Although yield reduction was significant only in plants infested during early stages of their growth, chemical management of potato psyllids needs to be continued until at least a week before harvest to minimize ZC impact on the tuber quality.