After last years disatrously wet summer we found ourselves contending with a new problem... Bud Rot (Xanthommonas sp.).
The bacterium that causes bud rot of canna enters through the stomata (tiny openings or pores, on the underside of leaves and used for gas exchange) of young leaves and flower buds, usually before they have opened. As the leaf unfolds, numerous spots are visible which enlarge and run together along the veins, sometimes giving the leaf a striped appearance.
The spots may at first be whitish, but soon turn to black. Infected flower buds may turn black and die before opening.
The disease may progress down the leaf stalk and kill the young stem and buds.
If older leaves are infected, the disease area spreads slowly, forming irregular, yellowish spots with water-soaked margins.
The bacteria live through the winter on the rhizomes. The disease occurs early in the season on young plants in hot-houses and gardens.
Control: Use only healthy rootstocks for propagation. Suspicious ones should be dipped in a streptomycin solution before planting. Streptomycin bud and leaf sprays might also help.
Avoid over-watering, overcrowding, and poor ventilation.