Powdery mildew is a highly destructive winter wheat pathogen in China. Since the causative agent is sensitive to changing weather conditions, we analyzed climatic records from regions with previous wheat powdery mildew epidemics (1970 to 2012) and investigated the long-term effects of climate change on the percent acreage (PA) of the disease. Then, using PA and the pathogen’s temperature requirements, we constructed a multiregression model to predict changes in epidemics during the 2020s, 2050s, and 2080s under representative concentration pathways RCP2.6, RCP4.5, and RCP8.5. Mean monthly air temperature increased from 1970 to 2012, whereas hours of sunshine and relative humidity decreased (P < 0.001). Year-to-year temperature changes were negatively associated with those of PA during oversummering and late spring periods of disease epidemics, whereas positive relationships were noted for other periods, and year-to-year changes in relative humidity were correlated with PA changes in the early spring period of disease epidemics (P < 0.001). Our models also predicted that PA would increase less under RCP2.6 (14.43%) than under RCP4.5 (14.51%) by the 2020s but would be higher by the 2050s and 2080s and would increase least under RCP8.5 (14.37% by the 2020s). Powdery mildew will, thus, pose an even greater threat to China’s winter wheat production in the future.

The Botryosphaeriaceae family is considered a fungal family that includes pathogens causing latent infection of woody plants, and a number of species were identified as causal pathogens of canker and shoot blight diseases. To better understand the process of latent infection of major canker-causing pathogens in woody tissues in different tree crops important in California, shoot and bud samples were randomly collected from four tree crops: almond, dried plum, pistachio, and walnut. The previously developed DNA primers and quantitative real-time PCR (qPCR) assay systems were applied to detect six canker-causing pathogen groups, including Botryosphaeria dothidea, and species of Cytospora, Diplodia, Lasiodiplodia, Neofusicoccum, and Phomopsis. The concepts of molecular severity (MS) and latent infection index (LII) were introduced and applied to quantify the latent infection levels for these samples. Variation in incidence of latent infection among pathogen groups was observed, whereas the incidences were relatively low among species of Phomopsis and Diplodia. High incidences of Cytospora spp. were observed in two dried plum (prune) orchards. Most orchards showed high incidences of B. dothidea and Lasiodiplodia spp. and moderate incidences of Neofusicoccum spp. Variations in MS were observed among samples of the studied orchards, ranging from 4 to 8. The overall results of LII demonstrated that species of Diplodia and Phomopsis were less important in population development of canker-causing pathogens at the latent phase. Lasiodiplodia spp. were the most aggressive and had been well developed in populations among the studied tree crops. Cytospora spp. became predominant in two of the three dried plum orchards, whereas B. dothidea and Neofusicoccum spp. showed trends of increase in incidence across various tree crops. This study also demonstrated the usefulness of this sensitive qPCR approach in providing evidence of the latent phase of major canker-causing pathogens of stone fruit and nut crops at an early stage of latent infection in woody plant tissues.

Alternaria heart rot of pomegranate fruit is generally considered a minor disease. However, the current listing of the causal pathogen in California as “Alternaria sp.” or as “A. alternata and other Alternaria spp.” restricts trade of the crop to some major export markets where quarantines are in place in an effort to prevent the spread of unknown or undescribed pathogens. Thus, species identification of the pathogen is critical in determining whether infected fruit should be regulated by quarantines. In this study, 86 isolates of Alternaria were collected from pomegranate fruit with Alternaria heart rot symptoms from major production areas in California. An unweighted pair group method with arithmetic means analysis based on amplified fragment length polymorphisms revealed two main clusters, each with a high degree of variability. One of the clusters contained 24 isolates from pomegranate and one reference isolate of Alternaria arborescens. Reference isolates of A. alternata and A. tenuissima were found among pomegranate isolates in the other cluster, and these two species could not be separated. In maximum-parsimony analysis of ribosomal DNA internal transcribed spacer sequence data, representative pomegranate isolates all clustered with reference sequences of species in section Alternaria. Single-nucleotide differences separated A. arborescens, A. gaisen, and two pomegranate isolates from the majority of the remaining isolates. Sequence comparisons of gapdh and OPA10-2 loci indicated that none of the pomegranate isolates were identical to A. gaisen, which is a quarantine pathogen. Due to high genetic similarity and variability of morphological characteristics, revisions of the taxonomy of small-spored Alternaria spp. have been proposed by others. Based on this recent taxonomic work and work herein by us, pomegranate isolates from California can be assigned to A. alternata and A. arborescens. Reassessment of export restrictions for California pomegranate due to the previous pathogen classification is warranted. Fruit injection inoculations with conidia of 12 representative isolates 3 months before harvest caused typical symptoms of Alternaria heart rot, whereas flower inoculations did not result in fruit disease.

Alternaria leaf spot caused by Alternaria alternata and A. arborescens is a common disease of almond in California. Succinate dehydrogenase inhibitors (SDHIs) are widely used for its management; however, we observed reduced performance of SDHI fungicides at some field sites. Thus, we evaluated the sensitivity to boscalid of 520 isolates of the main pathogen A. alternata collected from major production areas between 2006 and 2019, and also evaluated the sensitivity of a subset of 204 isolates to six members of the SDHIs belonging to six subgroups. Additionally, 97 isolates (14 sensitive and 83 with reduced sensitivity) of the 204 were used to determine the molecular mechanisms of resistance. A wide range of in vitro concentrations to effectively inhibit mycelial growth by 50% (EC₅₀ values) was determined for each fungicide using the spiral gradient dilution method. Some isolates were highly resistant (EC₅₀ values >10 μg/ml) to boscalid (a pyridine-carboxamide), pyraziflumid (a pyrazine-carboxamide), and fluxapyroxad (a pyrazole-4-carboxamide), but not to fluopyram (a pyridinyl-ethyl-benzamide), isofetamid (a phenyl-oxo-ethyl thiophene amide), and pydiflumetofen (a N-methoxy-(phenyl-ethyl)-pyrazole-carboxamide). There was no strong cross resistance among the fungicides tested, including for the two pyrazole-4-carboxamides fluxapyroxad and penthiopyrad (tested for 33 of the 204 isolates). The comparison of EC₅₀ values for fluopyram and isofetamid resulted in the highest coefficient of determination (R² = 0.582) among 10 pairwise comparisons between subgroups. Sequence analyses of the 97 isolates revealed five mutations in SdhB, SdhC, or SdhD subunits of the Sdh target gene among 73 isolates with reduced sensitivity to at least one SDHI. No mutations were detected in the 14 sensitive isolates and in 10 of the 83 isolates with reduced sensitivity. The most common mutation (59 isolates) was H134R in SdhC. Other mutations included H277Y (eight isolates) and H277L (two isolates) in SdhB, as well as G79R (two isolates) and S135R (two isolates) in SdhC. Mutations H277Y in SdhB and S135R in SdhC were only present in isolates collected in 2012 or earlier. Both conferred mostly high levels of resistance to boscalid and also reduced sensitivity to pyraziflumid, fluxapyroxad, and isofetamid with intermediate EC₅₀ levels. Mutations H277L in SdhB, as well as H134R and G79R in SdhC, found in isolates obtained after 2012 had very similar resistance phenotypes with different levels of resistance to boscalid, pyraziflumid, and fluxapyroxad, whereas sensitivity to fluopyram, isofetamid, and pydiflumetofen was mostly less affected. Our data for SDHI fungicides do not support the classical concept of positive cross resistance within a single mode of action. Because some mutations conferred resistance to multiple SDHI subgroups, however, resistance management needs to consider all SDHIs as a homogenous group that should be mixed or rotated with other modes of action to delay development of resistance.

Pistachio (Pistacia vera) is an important crop in Italy, traditionally cultivated in Sicily (southern Italy) for several decades now. In recent years, new orchards have been planted in new areas of the island. Field surveys conducted in 2019 revealed the presence of symptomatic trees showing shoot dieback, cankers, fruit spots, and leaf lesions. Isolations from symptomatic samples consistently yielded fungal species in the Botryosphaeriaceae family. Identification of collected isolates was conducted using morphological and molecular analyses. Morphological characterization was based on conidia measurements of representative isolates and also effects of temperatures on mycelial growth was evaluated. DNA data derived from sequencing the ITS, tef1-α, and tub2 gene regions were analyzed via phylogenetic analyses (maximum parsimony and maximum likelihood). Results of the analyses confirmed the identity of Botryosphaeria dothidea, Neofusicoccum hellenicum, and N. mediterraneum. Pathogenicity tests were conducted on detached twigs and in the fields both on shoots as well as on fruit clusters using the mycelial plug technique. The inoculation experiments revealed that among the Botryosphaeriaceae species identified in this study N. hellenicum (occasionally detected) and N. mediterraneum were the most aggressive based on lesion length on shoots and fruits. N. mediterraneum was the most widespread among the orchards while B. dothidea can be considered a minor pathogen involved in this complex disease of pistachio. Moreover, to our knowledge, this is the first report of N. hellenicum in Italy.