Bacterial spot is caused by Xanthomonas vesicatoria and is most noticeable in its effect on fruit. However it may also cause considerable injury to the foliage of seedlings and plants in the field. Bacterial spot occurs during rainy periods.
Symptoms of bacterial spot
Leaflets show small irregular dark spots of greasy appearance and the centres dry out and frequently tear. The stems of seedlings are also often spotted.
On older plants the spotting occurs on older leaves and there may be some defoliation. The disease is often confused with botrytis or early blight. Considerable blossom drop can also occur.
The easiest way to identify bacterial spot are the young green fruits. The tend to have small water-soaked spots which appear fits. Later they become raised and enlarge until they are ±3mm to 6mm in diameter. Initially the spots have white halos and may be confused with bird’s-eye spotting of bacterial canker. The white spot in bacterial canker is much more prominent than in bacterial spot. The most striking way to identify bacterial spot is that the spot is raised above the rest of the fruit area. The halo part is also raised. In a later stage of the disease development the halo disappears and the centre of the spot becomes irregular light brown and slightly sunken with a rough scabby surface. It is important to note that mature fruits are not infected.
Control of bacterial spot
Bacterial spot is difficult to control. The only known chemical to control the disease are copper based such as copper hydroxide, copper oxychloride or copper sulphate. Every effort should be made to ensure clean seedlings if they are purchased from a nursery.