SUMMARY Brown rot, caused by Monilinia fructicola, is the most important peach disease in Brazil. However, there is lit- tle information about the epidemiology and spatial pat- terns associated with this disease in organic orchards. This study characterized the spatial distribution of blos- som blight and brown rot within trees under natural in- fection. The study was carried out in 2007 and 2008 in an organic orchard located in southern Brazil. The spa- tial pattern of diseased flowers (in a sample of 80 flow- ers per tree per year) and fruit (from 64 to 309 fruit per tree per year) within each peach tree was assessed using 16 quadrats per tree. The dispersion index (D) and the modified Taylor’s power law were used to characterize the spatial pattern of the disease. The conidia density of M. fructicola and the weather conditions were moni- tored. In both years of assessment, diseased flowers were distributed at random in 90% of the trees, and no correlation was found between the incidence of blossom blight from a given year and the incidence of blossom blight or brown rot in the previous year. The distribu- tion of brown rot within the tree was aggregated in both years in 70% of the trees where disease incidence was lower than 90%, and a significant positive relationship was observed in disease incidence from one year to the other. Taylor’s power law was significant for blossom blight and brown rot but aggregation was verified only for brown rot. According to these results, blossom blight is not an important source of inoculum to fruits, and we suggest that the intensive chemical control dur- ing flowering in the subtropical regions of Brazil should be reviewed. Key words: epidemiology, Prunus persicae, spatial dis- tribution, aggregation. Corresponding author: L. Amorim Fax: +55.19.34344839 E-mail: lilian.amorim@usp.br INTRODUCTION Brown rot, caused by Monilinia fructicola (G. Win- ter) Honey, is the most important peach disease in Brazil (May De Mio et al., 2008). Blossoms and fruits are susceptible to infection (Byrde and Willetts, 1977). Symptoms of the disease appear initially on the blos- soms, which are covered with the grayish conidia of the fungus. At the base of infected flowers, small cankers can develop on the twigs around the flower stem; some- times, the cankers encircle the twigs and cause twig blight (Agrios, 2005). Fruit symptoms appear when they approach maturity as small, circular, brown spots that rapidly increase and cover the entire fruit surface. After- wards, fruits become completely rotted and dry up into a mummy (Agrios, 2005). Apothecia can develop from fruit mummies partially buried in the soil or debris on the orchard floor (Ogawa et al., 1995). In temperate re- gions, the inoculum from sexual reproduction has an important role in starting epidemics (Byrde and Wil- letts, 1977), but in Brazil, the sexual stage of M. fructico- la is rare (Martins et al., 2005). In this country, the in- oculum for primary infection probably comes from mummified fruit attached to trees from the same or- chard or from neighbouring orchards. It is also possible that the cankers produced on twigs in the previous year contribute to the primary inoculum in the next year as described Everhart et al. (2011) for cherry trees. Organic production systems represent 1.75% of Brazilian agriculture, but the domestic market for or- ganic products in Brazil is continuously increasing. On- ly 4.4% of the area given over to organic agriculture is occupied by fruit and vegetables (IBGE, 2006). One of the main problems in organic peach production is the control of diseases and pests. In the southern region of Brazil, where the amount of rain is usually high during development of the fruit, the average brown rot inci- dence from 2005 to 2008 in organic orchards ranged from 20 to 98% (Negri et al., 2011; Keske et al., 2011). In organic systems, brown-rot control is based on win- ter treatments with lime sulfur or Bordeaux mixture, sanitation measures, such as the removal of diseased twigs, branches and fruit during pruning operations, and fruit protection, by paper bagging each fruit indi- Journal of Plant Pathology (2013), 95 (1), 67-73 Edizioni ETS Pisa, 2013 67 SPATIAL PATTERN OF BROWN ROT WITHIN PEACH TREES RELATED TO INOCULUM OF MONILINIA FRUCTICOLA IN ORGANIC ORCHARD C. Keske 1 , L.L. May-De Mio 1 and L. Amorim 2 1 Department of Crop Protection, Federal University of Paraná, Rua dos Funcionários 1540, PR. 80.035.050 Curitiba, Brazil 2 Department of Plant Pathology, University of São Paulo, Caixa Postal, 09, SP. 13.418.900, Piracicaba, Brazil