Published in Phytopathology Journal online on Jan 12, 2022, by A.A. Wright, M. Shires, C. Molnar, G. Bishop, A. Johnson, C. Frias, S.J. Harper
Summarized by Corina F. Serban – ITT Extension Coordinator – Little Cherry Disease Extension and Outreach program, February 2022
The parallel outbreak of Little Cherry and X-disease has become a singular threat to the PNW cherry industry, causing the removal of ca. 1000 acres in the past five years. In Washington, one of the major pathogens is Candidatus (Ca.) Phytoplasma (P.) pruni is commonly known as X-disease phytoplasma (XDP). XDP infection cherries result in small, misshapen fruit with poor color and taste, making the fruit unmarketable. However, this disease has a long development cycle and a scattered, non-uniform symptom distribution in the early stages. The symptoms of XDP in cherries get worse over time.
To better understand the biology as well as the relationship between pathogen titer and disease expression, the Wright et al. 2022 study looked at the seasonal, spatial, and temporal examinations of ‘Ca. P. pruni’ titer and distribution in infected orchard-grown trees.
A sequential sampling at different growing stages (bloom, shuck fall, pit hardening, harvest, post-harvest, and pre-dormancy) of heavily infected trees showed marked seasonal patterns. Wright et al. 2022 observed that titer in woody stem tissues was consistently higher throughout the season, particularly between bloom and harvest, and lower between postharvest and pre-dormancy. This was in contrast to leaf tissue, which was low at bloom and increased through the season, peaking at pre-dormancy (September) (Figure 1a). These data would suggest that leafhopper feeding late in the season is more likely to result in acquisition. In generative tissues, the pattern was different, with ‘Ca. P. pruni’ being detected at low titer at bloom in the flower
stem, which increased as it developed into the fruit pedicel through to harvest (Figure 1b).
Figure 1: Quantification of XDP in (A) woody stems and leaves versus (B) pedicel and fruit
This study also showed that mapping XDP distribution and titer in trees at different stages of infection indicated that infection proceeds through a series of stages. Initially, the infection spreads from where the leafhoppers feed, accumulates in the roots and lower trunk, then moves up to the aerial parts of the tree, with infection of specific tissues and limbs followed by an increasing phytoplasma titer (Figure 2a and 2b).
Figure 2: Mapping XDP titer and symptom severity on cherry trees at different stages of infection.
Finally, the Wright et al. 2022 study looked at whether there was a within-tree correlation between symptom severity and ‘Ca. P. pruni’ titer. This study observed that titers in limbs with asymptomatic, mild, and moderately symptomatic fruit were not significantly different. In contrast, limbs with fruit expressing severe symptoms contained significantly higher titer.
The data presented in Wright et al. 2022 study can aid in targeting the most optimal tissues to sample for diagnostic testing at different times of the year, particularly before visual symptoms appear. For example, in collecting woody stem tissue instead of leaves prior to harvest, and the latter postharvest to pre-dormancy, and in all cases, sampling lower down the tree.
A.A. Wright, M. Shires, C. Molnar, G. Bishop, A. Johnson, C. Frias, S.J. Harper. Titer and distribution of Candidatus phytoplasma pruni in Prunus avium. Phytopathology. 2022 January 12. https://doi.org/10.1094/PHYTO-11-21-0468-R.
Scott J. Harper
Clean Plant Center Northwest
WSU-IAREC, Prosser, 99350
Office: Hamilton Hall 201
Tel: (509) 786-9230
Fax: (509) 786-9370
Fruit Matters articles may only be republished with prior author permission © Washington State University. Reprint articles with permission must include: Originally published by Washington State Tree Fruit Extension Fruit Matters at treefruit.wsu.edu and a link to the original article.