Outbreaks of bacterial canker (Pseudomonas syringae) are less frequent now as seeds are produced under more hygienic conditions. Hybridisation has raised seed production costs, so more care is taken in choosing sites and monitoring hygiene. Nevertheless, the disease can still be introduced through infected seed. Soak suspect seed in water of 55ºC for 30 minutes and sow soon afterwards. There are chemical treatments, but these aren’t effective if the bacteria are systemically in the seed. The disease is most active at about 25ºC.
The symptoms vary and can be confused with diseases like Fusarium, so they’re worth describing in detail:
a) One side of the leaf wilts while the other side is visually healthy, or there is die back. b) Yellow or dark streaks along stems or petioles (leaf stems).
c) Cankers on stems, petioles or leaf midribs.
d) When stems are cut lengthwise, there will be yellow or reddish discolouration of the pith and hollow areas.
e) Spots on the fruit – dark, raised areas with a distinct light halo like a bird’s eye.
f) Brown, pimple-like spotting of peduncles to which fruit are attached.
g) A yellowish bacterial ooze can be squeezed out of the infected stem tissue.
h) No chlorosis (yellowing) on the foliage.
i) discolouration of the roots.
j) Roots may start forming on the lower stem.
l) Leaves look burnt or dried out. he disease can occur both outdoors and in greenhouses. Fruit symptoms usually only manifest after rain, so they’re unusual in greenhouses.
Copper sprays can slow the disease down, but the act of spraying also disseminates the bacteria, which enter where hairs are broken by high pressure spraying. gentle, fine spray is preferable. he disease can contaminate soil for five years in extreme cases and survive in compost for two. It also lasts 10 months on stakes and equipment. I n infected greenhouses, the soil should be fumigated and surfaces sprayed with a suitable disinfectant. Infected land should be well ploughed to bury all infected material. here are varieties available with, some resistance, which is really only a solution in areas experiencing persistent outbreaks. – Bill Kerr ((016) 366 0616 or e-mail [email protected]). |fw
