Brown spot disease in ‘Rocha’ pear Portuguese orchards

Brown spot, caused by Stemphylium vesicarium, is the main fungal disease of pear causing severe crop losses. Disease incidence was evaluated in flowers and fruits throughout the growing season in two orchards with moderate (MI) and low (LI) disease incidence history, both of them with two different weed control systems, mechanical cultivation (M) and natural cover (C). In both orchards. vegetation in the tree strip was controlled with herbicide. Selected isolates were used to fulfill pathogenicity tests. These tests were performed in a commercial orchard, without the disease. Fruits without lesions were inoculated with mycelial discs of a total of 15 isolates. Controls were inoculated with PDA discs. Flowers’ disease incidence on May 16 (BBCH 65) in LI-C orchard was lowest (31.7%) differing significantly from the other treatments. Sepals, styles and stigma were the most infected organs, however, the pathogen was also isolated from ovaries and stamens. Regarding fruits’ disease incidence, on the May 30 (BBCH 72), LI-M showed the lowest incidence (20.0%) and MIC, the highest (56.7%) meaning that infection in the initial stages of fruit growth is strongly influenced by natural cover (inoculum source). However, on July 15, orchard history disease incidence was dominant with 51.7% of the fruits in MI C and M average was symptomatic whereas in LI-C and M only 17.5% average showed symptoms. Fruit disease prevalence was also lower in the orchards LI history. Results from pathogenicity tests revealed that 53.4% of the isolates were pathogenic. Among them 25% of the isolates obtained from flowers and 75% of the isolates obtained from fruits produced typical brown spot symptoms. The most virulent isolates were obtained from fruits, but there was no relation with orchard treatments. These results show that cultural control measures based on inoculum reduction are of utmost importance and that an isolate’s pathogenicity is a genetic feature independent from the orchards