Bacteria can spread around plants through water, so infections tend to become worse following wet or humid weather. Initially, the bacteria begin to multiply on the surfaces of leaves, fruits and other external plant tissues. The second stage involves bacteria entering the plant through wounds or natural openings, such as the leaf stomata (pores). From here, the disease spreads via fallen plant matter and seed. The latter can be transported further via wind, rain and agricultural machinery. Animal and insects can sometimes be vectors, but this is less likely.