Apple blotch (Marssonina blotch), caused by Diplocarpon coronariae (synonyms: D. mali; Marssonina coronaria) is a major disease of cultivated apples in Japan. Here we tested the usefulness of weather data to predict the start of primary infection and symptoms in relation to data for ascospore discharges and apple growth stages. We analyzed 18 cases of the 1st peak of ascospore dispersal (“1st peak”) among seven prefectures in Japan during 1998–2020. All the 1st peaks occurred on rainy days after bloom started for cv. Fuji, and on average, they coincided with the petal fall stage. A total 76 rain events that occurred from start to 1st peak were examined (1st peak: 18 cases; nonpeak: 58 cases). Logistic regression analyses revealed that, of three models examined, the wetness duration model was the best fit model based on Akaike information criterion. A final logistic regression model only comprised the interaction variable between average temperature (≧13.8°C) and duration (≧6 h) during wetness. In the final model, the rain events could be divided into 1st peak and nonpeak with 97% accuracy. The thermal time required for a latent period during 90% of total apple blotch lesions after conidial inoculation was estimated as 159 degree-days (base = 4.6°C). Both date of first infection and first symptom found in the field exposure and survey corresponded almost exactly to the predicted dates using both the 1st peak model and the latent period. These results suggest that weather data can be used to estimate both the start of primary infection and symptoms to analyze epidemic trends for apple blotch.

The tobamovirus tomato brown rugose fruit virus (ToBRFV) causes leaf mosaic and severe fruit symptoms such as browning and distortion (rugosity) on solanaceous crops. Since the first report of ToBRFV in Israel in 2014, it has been found in nearly 40 countries. To obtain further insights on its host range and pathogenicity, detection and preventing its invasion into Japan, we inoculated varieties of solanaceous crops and weeds in a glasshouse with the Israeli ToBRFV isolate and evaluated local and systemic symptoms and infection of the non-inoculated upper leaves. All the tomato varieties tested except for GCR237, homozygous for tobamovirus resistance gene Tm-1, were infected systemically and had mosaic symptoms on leaves. However, rugosity on fruits was very rarely observed on any varieties in our experimental conditions. Bell pepper varieties harboring L¹, L², L³, and L⁴ were resistant or immune to systemic infection. Two Japanese eggplant varieties were systemically infected, but they developed only temporary faint, chlorotic spots and wavy leaves. Most of the other solanaceous crops and weeds tested were also systemically infected by ToBRFV, including newly identified hosts Solanum sisysmbriifolium, S. muricatum, S. ptycunthum, S. nigrescens, and Nicandra physalodes. These results indicate that ToBRFV has a considerably wide host range especially in the Solanaceae.

Tomato brown rugose fruit virus (ToBRFV, a tobamovirus) causes mosaic and severe fruit symptoms on tomatoes and solanaceous crops. Since its discovery in the Middle East a decade ago, ToBRFV has been found in nearly 40 countries. Although it has not yet been reported in Japan, control measures are urgently needed to prepare against ToBRFV. In this study, we tested disinfestation procedures that are already used against tobamoviruses in Japan for their efficacy against ToBRFV: (i) chemical disinfestation of equipment (scissors), (ii) chemicals and dry heat disinfestation of ToBRFV-contaminated tomato seeds, and (iii) inoculation of tomato seedlings with an attenuated strain of tomato mosaic virus (ToMV) to inhibit infection and symptom development. The control measures generally used against tobamoviruses were also effective against ToBRFV on tools and seeds, but the measures differed in efficacy and advantages. The attenuated ToMV was ineffective. Precautions for using these treatments and possible solutions or improvements are discussed.