Effects of Potato Psyllid Vector Density and Time of Infection on Zebra Chip Disease Development after Harvest and during Storage

Abstract

Candidatus Liberibacter solanacearum" (Lso) (="Candidatus Liberibacter psyllarous") is an uncultured, phloem-limited bacterium that is associated with zebra chip disease (ZC) in potato (Solanum tuberosum L.) and transmitted by the potato psyllid (Bactericera cockerelli (Šulc)). Vector density and timing of infection have been shown to affect ZC prevalence at harvest; however, little work has been done on disease development during storage. Here we confirm with field-cage trials that ZC prevalence at harvest was greater with increased time between inoculation and vine kill. Moreover, we show that with Pacific Northwest growing conditions, ZC can develop over time during storage. Plants inoculated 2 to 3 weeks before vine kill showed little or no ZC symptoms in tubers at harvest, but higher prevalence of symptoms after 3 months in storage. For plants inoculated at 4 to 5 weeks before vine kill, tubers exhibited notable symptoms at harvest, but still showed evidence of symptom development after storage. Plants inoculated within 1 week before vine kill exhibited little or no risk of ZC in tubers at harvest or after storage. Higher vector density tended to contribute to ZC prevalence, but was far less important than timing of infection. These results underscore the potential danger of underestimating ZC prevalence at harvest for tubers being stored long term, and suggest that plants at risk of Lso infection should be protected from potato psyllids until at least 2 weeks before vine kill.

Effect of the level of “Candidatus Liberibacter solanacearum” infection on the development of zebra chip disease in different potato genotypes at harvest and post storage

“…Conversely, in Idaho, late-season infections are likely to occur [41], a pattern that may correspond to the increase in the number of potato psyllids captured in the field late in the season [44]. Time of infection is known to affect both the severity of ZC symptoms [17,18] and the Lso quantity within host plants. Likewise, our results showed that ZC symptom severity and Lso titer vary with the time of infestation (or infection).…”

“…At 4 DBVR, symptoms were not detectable at harvest. Although successful Lso translocation from leaves to tubers has been reported in potato plants infected 4 days before harvest [19], ZC symptom may not be observed for infestations that occur up to one week before harvest [15,17]. Lso detection success may also be limited in the late infestation times of 4 and 12 days before vine removal, as pathogen quantities are at undetectable levels; many tubers that are infected late in the season may proceed to test positive after a few months of storage [19].…”

“…The severity of ZC symptoms in tubers has been shown to be positively correlated with variations in levels of certain phenolic compounds, reducing sugars and defense enzymes [14,15]. Likewise, ZC symptom severity is known to be influenced by the infection time and host plant physiological responses [10,16,17]. Previous studies have shown that early-season infections produce severe symptoms in tubers whereas symptoms in late-season infections may not be visible in the tubers [15,17,18].…”

“…Likewise, ZC symptom severity is known to be influenced by the infection time and host plant physiological responses [10,16,17]. Previous studies have shown that early-season infections produce severe symptoms in tubers whereas symptoms in late-season infections may not be visible in the tubers [15,17,18]. Recent studies have suggested that disease continues to impact tubers postharvest, and during the process of storage [17,19], a finding which is particularly relevant to the late-season infected potatoes where the tubers appear asymptomatic at the time of harvest [10,15,17,18].…”

“…Previous studies have shown that early-season infections produce severe symptoms in tubers whereas symptoms in late-season infections may not be visible in the tubers [15,17,18]. Recent studies have suggested that disease continues to impact tubers postharvest, and during the process of storage [17,19], a finding which is particularly relevant to the late-season infected potatoes where the tubers appear asymptomatic at the time of harvest [10,15,17,18]. Lso acquisition success by potato psyllids may be affected by the distribution of Lso within the source plant, as it is known to be very heterogeneous [20,21].…”