The ecology of infection between a transmissive and a dead‐end host provides clues for the treatment of a plant disease

Abstract

Abstract. In plant pathosystems in which some hosts are transmissive and some are dead-ends, infection is mediated by multiple factors including the susceptibility of both hosts, the sporulation potential of transmissive hosts, the mobility of infectious propagules, the presence of environmental factors conducive to infection, and the variability in distribution of both host types. The factors above were studied for the California forest disease sudden oak death caused by the pathogen Phytophthora ramorum. This pathogen is exotic to California, and while it sporulates at significant levels on the leaves of California bay laurels, four susceptible oak species appear to be non-infectious dead-end hosts. Here, we report, for the first time, on inoculum levels necessary to successfully infect adult oaks and on the distribution of such inoculum levels through time and space thanks to a seven-year-long monitoring effort across a network of 128 monitoring points. Through a series of geostatistical and statistical analyses, we show that the presence of high inoculum loads is positively correlated with close proximity to bay laurels, with high rainfall levels, and with warmer temperatures. Data are consistent with splash dispersal of the pathogen and show that increased presence of tanoak corresponds to a reduced presence of bay laurels and to a lower frequency of high inoculum events. Removal of bay laurels resulted in a substantial decrease of number of events in which spore loads were high enough to infect oaks. This effect was significant when bays were removed 10 m around sampling points, thus indicating that removal of bays 10 m around oaks is a valid approach to reduce infections of oaks.

Environmental Factors Driving the Recovery of Bay Laurels from Phytophthora ramorum Infections: An Application of Numerical Ecology to Citizen Science

“…Although the number of recovered trees was assessed by an algorithm, it is worth noting that recovery from P. ramorum is not simply a theoretical possibility, but a fact that has already been reported, based on repeated sampling of the same set of trees [46]. The large recovery rate detected among bay laurels is in agreement with the notion that this species is a transmissive host of P. ramorum, acting as a reservoir of inoculum and supporting sporulation of the pathogen, but with infection limited to leaves without ever spreading even to the twigs [3,22,34,38,40]. P. ramorum is not associated with severe symptoms, relevant physiological disequilibrium, or mortality on bay laurels [39,84,85].…”

“…The role of environmental factors in boosting the spread of P. ramorum has been investigated in several previous studies [17,22,32,[42][43][44][45][46]48,85], yet this is the first attempt to elucidate their possible effects on host recovery. Correlation analyses showed high colinearity among environmental factors.…”

“…When rainfall values are less than 1000 mm, the likelihood of recovery increases steeply, exceeding 50% when precipitations drop below 500 mm. Although P. ramorum is an atypical Phytophthora species, being a pathogen on aerial parts of its hosts, its sporulation and spread at local scale is largely dependent on the occurrence of abundant rainfalls and on the presence of running water and wet surfaces, as largely documented [17,22,32,38,[40][41][42][43][44][45][46]48]. By reducing loads of infectious inoculum, aridity could hinder P. ramorum and promote the recovery of bay laurels through the interruption of reinfection cycles and leaf drop.…”

“…A prolonged aridity has been associated with the absence of novel P. ramorum symptoms in bay laurels [85], but at the same time aridity is likely to be well tolerated by bay laurels, trees whose ecological plasticity and adaptability to a wide range of habitats, climate and soil types have been largely documented [92]. Interestingly, a recent study [22] disclosed the causal relation between rainfalls and the occurrence of events in which P. ramorum sporulation was high enough to reach the threshold necessary to infect oaks. Such events tend to be rare with low precipitation levels, thus we cannot exclude that this phenomenon could be associated with an increased recovery among bay laurels.…”

“…The interest of forest pathologists in unraveling environmental factors driving plant diseases has been amplified in the last decades by the onset of relevant epidemics caused by emerging pathogens such as Phytophthora ramorum Werres, De Cock and Man in't Veld in Western North America, Heterobasidion irregulare Garbelotto and Otrosina, Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz and Hosoya, and Gnomoniopsis castaneae G. Tamietti in Europe, just to cite a few relevant examples [3][4][5][6][7][8][9][10][11][12]. The main environmental drivers underlying the success of such novel epidemics have often been identified through a numerical ecology approach, based on computational and multivariate statistical techniques suitable to deal with complex ecological datasets [10,11,[13][14][15][16][17][18][19][20][21][22].…”