The frequency of modification of Dothistroma pine needle blight severity by fungi within the native range

Foliar disease is most often understood as the product of the interaction between host and pathogen in an abiotically conducive environment. However, other microbes within the foliar microbiome may increase or decrease disease severity as either pathogen enablers or antagonists. The frequency and extent of such disease modification in nature are largely unknown, but modification is likely to occur at high frequency in the native range of a pathogen. In this study, we provided an opportunity for six fungi to modify the severity of Dothistroma needle blight in Pinus ponderosa within part of the native range of the disease near Priest River, Idaho, USA. Five fungi were common members of the P. ponderosa microbiome in the region, and one was isolated from co-occurring Pseudotsuga menziesii. Inoculum of each of the six was applied to an average of 187 newly emerging needles in the candles (new shoots) of each of ten, seven-year-old trees; control candles were treated with sterile distilled water. Disease severity per needle (i.e., lesion length/total needle length) was determined one year later following natural infection and development of Dothistroma needle blight in a total of 13,085 needles. Five of the six fungi significantly modified disease severity; the overall model (“inoculant”, “tree”, and “inoculant x tree” as predictors) explained 53% of the variation in disease severity. Penicillium goetzii was the sole antagonist and reduced disease severity by nearly 7% compared to control needles. Four of the fungi (Sydowia polyspora, Bionectria ochroleuca, Penicillium raistrickii, and a culturable species of Elytroderma) acted as pathogen enablers, increasing disease severity 4.7%, 4.2%, 3.6%, and 2.5%, respectively. Our results show that microbial modification of expression of Dothistroma needle blight in nature may be common within the native range of the pathogen. However, with more thorough sampling of the P. ponderosa microbiome, the extent of modification may go beyond the modest effects reported here for just a few members of the microbiome of the host and its forest community.