Survey of the factors causing fruit russet on Golden Delicious apple in (Semirom) (Short report)

Fruit russeting has been recently widespread on apple fruits cultivar Golden Delicious, resulting in decreased quality and value, in Semirom, Isfahan province, Iran Two groups of fruit russetting have been reported on the Golden Delicious cultivar. The first group has a genetic origin (Falginella et al., 2015) and the second group is related to environmental conditions, physical, mechanical and chemical factors, pests, pathogens and management practices. The amount of russet is affected by various factors such as weather conditions (rainfall, coldness, relative humidity, light quality on the fruit surface), chemicals (nutrients, hormones or fungicides), mechanical injury and pests' damage (Goffinet et al., 2002). In addition, fruit russet could be formed by epiphytic bacteria or yeasts (Goffinet et al., 2002; Heidarianet al. 2020), causative agent of apple powdery mildew (Podospha eraleucotricha (Ellis & Everh.) E. S. Salmon) and apple rust mite, Aculus schlechtendali.  The present study aimed to investigate the factors affecting the occurrence and development of fruit rust on Golden Delicious apple cultivar. Therefore, random sampling was performed from three major cultivation regions of Semirom city, having different climates. In each region three orchards were selected and in each orchard, 50 fruits were sampled three times in two consecutive years, from the time of symptom occurrence until harvest time. Samples were inspected in the laboratory in terms of rust type, percentage of russetted fruits (RI%) and fruit russeting index (FRI), based on predetermined scales. The results showed that based on the typical symptoms, there are four types of rust as follows on apple fruit in Semirom orchards. 1) Caused by a yeast-like fungus: It occurs in the stem-end cavity of the fruit, forming a brown fluffy tissue. The pathogenicity of this agent was proven (Heidarian et al. 2020). 2) Rust caused by powdery mildew: It is tan or gray color and has a mesh-like appearance. 3) Physiological rust: It is a type of sunburn called brown sunburn. 4) Rust caused by spraying chemicals such as fertilizers and pesticides: It usually occurs in places of the fruits where there is a possibility of accumulation of particles and is usually associated with burning of foliage. Mean Comparison indicated that the highest percentage of russetted fruits (73.25%) and fruit russeting index (37.7%) belonged to the yeast-like fungus Aureobasidium pullulans and the lowest of these values, 1.66% and 1.45%, respectively belonged to chemical spraying. Moreover, the results showed that 80% of Golden Delicious fruits have russetted with varying degrees of contamination, of which 53.6% were russetted by one factor and 36.83% by two or more factors.