Japanese quince (Chaenomeles japonica) is cultivated as a fruit crop because its fruit possesses valuable properties. The total area of Japanese quince in Latvia is continuously increasing; therefore, fruit rot can become an important reason of yield loss. The aim of this study was to clarify the fruit rot incidence in Japanese quince plantations in Latvia and explore the influencing factors of rot development in the year 2017. The evaluation of fruit rot was carried out during the fruit development in eight places of different regions of Latvia where Japanese quince is grown commercially. Three plantations of different Japanese quince cultivars in Dobele and seven plantations where Japanese quince is grown from seedlings were observed. Fruit rot incidence among plantations had no significant differences. Statistically significant differences in fruit rot incidence depending on quince growth stage (p is less than 0.05) were found. Significant differences in fruit rot incidence were found between cultivars in Dobele.

Japanese quince (Chaenomeles japonica) is a minor but prospective fruit crop due to the value of fruits and suitability for processing. Research aimed to clarify the incidence of quince leaf and fruit diseases depending on the year and plantation and determine the critical periods of diseases. The study was conducted from 2017 to 2019 in eight commercial Japanese quince plantations across different regions of Latvia. The incidence (%) of leaf and fruit spots and fruit rot was assessed several times during vegetation period. Considerable rot development was observed in only the first year of the study. The incidence of fruit rot in the middle of 2017 ranged from 0.7 to 11.7% depending on the plantation, while in 2018–2019 did not exceed 2% in any of the plantations. The critical period of fruit rot development was determined in the middle of fruit development and time closer to ripeness. Development of fruit spots increased closer to ripeness. In 2017, the incidence of fruit spots fluctuated from 0.7 to 44.0%, in 2018 – 7.7 to 46.0%, and in 2019 – 0.7 to 24.0%, depending on the plantation. The incidence of leaf spots reached high levels at the end of the vegetation periods (21–100% depending on year and plantation). The most critical periods of leaf spot development were the end of fruit development and full ripening of fruits. Determination of the critical periods of quince disease development could be used to create an effective and environmentally friendly disease control system.