|TWOMEY, MEGAN - Oregon State University|
|WOLFENBARGER, SIERRA - Oregon State University|
|WOODS, JOANNA - Oregon State University|
|Gent, David - Dave|
Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2015
Publication Date: 3/26/2015
Citation: Twomey, M.C., Wolfenbarger, S.N., Woods, J.L., Gent, D.H. 2015. Development of partial ontogenic resistance to powdery mildew in Hop cones and its management implications. PLoS One. 10(3):e0120987.
Interpretive Summary: Understanding how plant diseases cause damage to crops is of utmost importance for developing appropriate control measures. In this study, we conducted multiple experiments that demonstrated that the susceptibility of hop cones to powdery mildew varies depending on developmental stage. Disease severity and quality defects were most substantial when cones were attached at or within 21 days after bloom. Susceptibility of cones to powdery mildew decreased progressively after the transition from bloom to cone development, although complete immunity to the disease failed to develop. Even more, when cones were attacked by powdery mildew the frequency of colonization of by weak pathogens or secondary organisms was increased. Based on this knowledge, we demonstrated that under field conditions with a moderately susceptible cultivar it is possible target control meaures only to the juvenile stages of cone development. The findings indicate that the efficacy of a fungicide program may depend largely on timing of a small number of sprays during a relatively brief period of cone development.
Technical Abstract: Knowledge of processes leading to crop damage is central to devising rational approaches to disease management. Multiple experiments established that infection of hop cones by Podosphaera macularis was most severe if inoculation occurred within 15 to 21 days after bloom. This period of infection was associated with the most pronounced reductions in alpha acids, cone color, and accelerated maturation of cones. Susceptibility of cones to powdery mildew decreased progressively after the transition from bloom to cone development, although complete immunity to the disease failed to develop. Maturation of cone tissues was associated with multiple affects on the pathogen manifested as reduced germination of conidia, diminished frequency of penetration of bracts, lengthening of the latent period, and decreased sporulation. Cones challenged with P. macularis in juvenile developmental stages also led to greater frequency of colonization by a complex of saprophytic, secondary fungi. Since no developmental stage of cones was immune to powdery mildew, the incidence of powdery mildew continued to increase over time and exceeded 86% by late summer. In field experiments with a moderately susceptible cultivar, the incidence of cones with powdery mildew was similar when fungicide applications were made season-long or targeted only to the juvenile stages of cone development. These studies establish that partial ontogenic resistance develops in hop cones and may influence multiple phases of the infection process and pathogen reproduction. The results further reinforce the concept that the efficacy of a fungicide program may depend largely on timing of a small number of sprays during a relatively brief period of cone development. In practice, targeting fungicide and other management tactics to periods of enhanced juvenile susceptibility may be complicated by a high degree of asynchrony in cone development and other factors that are situation-dependent.