RESUMO
Twenty-eight cranberry farms in southwestern British Columbia were investigated for the prevalence and spatial distribution of fungal pathogens that contribute to fruit rot incidence. Farms were selected from six regions where most cranberry production is concentrated. Flowers, and green and ripe fruit (var. 'Stevens') samples, collected during two consecutive crop seasons, were analyzed for fruit rot pathogens. The most frequently isolated pathogens were identified as Allantophomopsis cytisporea, Botrytis cinerea, Coleophoma empetri, Colletotrichum fioriniae, Colletotrichum gloeosporioides, Fusicoccum putrefaciens, Glomerella sp., Phomopsis vaccinii, Phyllosticta elongata, Phyllosticta vaccinii and Physalospora vaccinii. The pathogens Allantophomopsis cytisporea, Phyllosticta spp., and Physalospora vaccinii were found at high incidence. These pathogens were present in all cranberry growing regions, although their mean percentage incidence varied from farm-to-farm and region-to-region. Amongst the pathogens from three phenological stages of cranberry crop examined, ripe fruit had the highest percentage incidence of fruit rot pathogens compared to that of flowers or green fruit; thus, indicating their presence at the early stages of crop development. The efficacy to inhibit the mycelial growth and spore germination of the fruit rot pathogens by twenty six fungicides, belonging to nine different modes of actions, were evaluated in vitro. The copper-based fungicides and captan of group M, flutriafol, triforine, difenoconazole, prothioconazole and propiconazole of group 3, benzovindiflupyr of group 7, and fosetyl-Al of group 33 demonstrated a high degree of efficacy in inhibiting the mycelial growth of all fruit rot pathogens. The fungicides chlorothalonil of group M, fenbuconazole of group 3, pyrimethanil and cyprodinil of group 9, and fludioxonil of group 12 also demonstrated activity against most fruit rot pathogens. The copper-based fungicides, chlorothalonil, captan, flutriafol, triforine, difenoconazole, prothioconazole, propiconazole, benzovindiflupyr, and fosetyl-Al effectively prevented the spore germination of most fruit rot pathogens. This demonstrated activity of the fungicides towards cranberry fruit rot pathogens should be assessed for efficacy in planta under field conditions. The current study identified the most critical fungal pathogens causing fruit rot of cranberry in British Columbia and potential fungicides that could be used in the management of fruit rot and to improve fruit quality and yield.
RESUMO
British Columbia (BC) is the lead producer of sweet cherries in Canada with more than 2,000 ha in production and a farm gate value of over CAD$100 million annually. Since 2010, an outbreak of little cherry disease caused by Little cherry virus 1 (LChV1) and Little cherry virus 2 (LChV2), as well as X-disease (XD) caused by 'Candidatus Phytoplasma pruni' has caused significant economic losses in neighboring Washington State (WA), USA. LChV1 and LChV2 have long been known to occur in BC (Theilmann et al. 2002); however, 'Ca. P. pruni' has not yet been reported in BC. Due to its geographical proximity to WA State, the BC cherry industry expressed significant concerns about the possible presence of the phytoplasma in cherry orchards. Accordingly, the main objective of this study was to survey cherry orchards to determine whether 'Ca. P. pruni' was present in symptomatic trees in BC. A total of 118 samples of leaves and fruit stems from individual symptomatic trees were collected prior to harvest from nine cherry orchards and one nectarine orchard in the Okanagan and Similkameen Valleys in BC. Characteristic symptoms included small and misshapen fruit with poor color development. Samples were submitted to AGNEMA, LLC (Pasco, WA) for testing using qPCR TaqMan assays for LChV1 (Katsiani et al. 2018), LChV2 (Shires et al. 2022) and 'Ca. P. pruni' (Kogej et al. 2020). Test results showed 21 samples (17.8%) from three cherry orchards positive for LChV2 and 2 samples (1.7%) from one cherry orchard positive for 'Ca. P. pruni'. In order to confirm the identification of 'Ca. P. pruni', part of the 16S ribosomal RNA gene was amplified by nested PCR using the P1/P7 followed by R16F2n/R2 primer sets (Gundersen and Lee 1996) and Sanger sequenced. BC-XD-Pa-1 (GenBank Acc. No. OR539920) and BC-XD-Pa-2 (OR537699) were identical to one another and showed 99.92% identity to the 'Ca. P. pruni' reference strain CX-95 (JQ044397). Analysis using iPhyClassifier (Zhou et al. 2009) indicated that they were 16SrIII-A strains. Interestingly, the two partial 16S sequences showed 100% nucleotide identity to strain 10324 (MH810016) and others from WA. For additional confirmation, partial secA (Hodgetts et al. 2008) and secY (Lee et al. 2010) translocases were amplified and sequenced. As with the 16S sequences, secY sequences (OR542980, OR542981) showed 99.92% nucleotide identity to strain CX-95 (JQ268249), and 100% to strain 10324 (MH810035). The secA sequences (OR542978, OR542979) had nucleotide identities of 99.77% to strain CX (MW547067), and 100% to the Green Valley strain from California (EU168733). Accordingly, 'Ca. P. Pruni' was confirmed to be present in sweet cherry samples from BC. 'Ca. P. Pruni'-related strains have been previously reported to occur in Canada in commercial poinsettias (Euphorbia pulcherrima) (Arocha-Rosete et al. 2021). To our knowledge, this is the first report of 'Ca. P. Pruni' in sweet cherry in Canada. Due to the important economic value of sweet cherries in BC, these findings are highly significant and represent the first steps towards the development of a surveillance system for early detection of XD, and consequent implementation of management strategies, including vector control. As required by federal and provincial regulations, cherry trees infected with LChV2 and 'Ca. P. Pruni' found in the survey were removed by the growers.
RESUMO
The leaf colonization strategies of two bacterial strains were investigated. The foliar pathogen Pseudomonas syringae pv. syringae strain B728a and the nonpathogen Pantoea agglomerans strain BRT98 were marked with a green fluorescent protein, and surface (epiphytic) and subsurface (endophytic) sites of bean and maize leaves in the laboratory and the field were monitored to see if populations of these strains developed. The populations were monitored using both fluorescence microscopy and counts of culturable cells recovered from nonsterilized and surface-sterilized leaves. The P. agglomerans strain exclusively colonized epiphytic sites on the two plant species. Under favorable conditions, the P. agglomerans strain formed aggregates that often extended over multiple epidermal cells. The P. syringae pv. syringae strain established epiphytic and endophytic populations on asymptomatic leaves of the two plant species in the field, with most of the P. syringae pv. syringae B728a cells remaining in epiphytic sites of the maize leaves and an increasing number occupying endophytic sites of the bean leaves in the 15-day monitoring period. The epiphytic P. syringae pv. syringae B728a populations appeared to originate primarily from multiplication in surface sites rather than from the movement of cells from subsurface to surface sites. The endophytic P. syringae pv. syringae B728a populations appeared to originate primarily from inward movement through the stomata, with higher levels of multiplication occurring in bean than in maize. A rainstorm involving a high raindrop momentum was associated with rapid growth of the P. agglomerans strain on both plant species and with rapid growth of both the epiphytic and endophytic populations of the P. syringae pv. syringae strain on bean but not with growth of the P. syringae pv. syringae strain on maize. These results demonstrate that the two bacterial strains employed distinct colonization strategies and that the epiphytic and endophytic population dynamics of the pathogenic P. syringae pv. syringae strain were dependent on the plant species, whereas those of the nonpathogenic P. agglomerans strain were not.
