Cauliflower stunt associated with a phytoplasma of subgroup 16SrIII-J and the spatial pattern of disease

Abstract

Since 2000, a disease has occurred with high levels of incidence in crops of cauliflower grown in the green belt area of the city of Sao Paulo, Brazil. The symptoms are characterized by stunting, malformation of the inflorescence, reddening leaves, and vascular necrosis, suggesting infection by phytoplasma. These symptoms are similar to those described in Brassicas species affected by the aster yellows (16SrI) group of phytoplasma. In the present study, a phytoplasma from the 16SrIII-J subgroup was identified in cauliflower plants based on actual and virtual RFLP patterns and phylogenetic analysis, and was distinct from the phytoplasmas frequently associated with aster yellows disease in Brassicas. Pathogenicity assays using dodder confirmed that the identified phytoplasma is the agent of the observed disease, which is here designated as cauliflower stunt. Consequently, this species of Brassica may be recognized as a new host for subgroup 16SrIII-J, which has frequently been found in diverse species cultivated in Brazil. The spatial pattern of diseased plants was determined in ten cauliflower plots of 300 to 728 plants each. All plants in these plots were evaluated by visual assessments, assigned as diseased or healthy and mapped. The dispersion index and Taylor’s power law were determined for various quadrat sizes and the results showed that the diseased plants were distributed in a random pattern in fields with a low disease incidence and in an aggregated pattern in fields with a disease incidence greater than 25 %. According to an isopath area analysis, diseased plants were predominantly present in the field borders, suggesting that the pathogen is possibly introduced by vector(s) from the external area.