Growths on Kowhai tree explained
RECENTLY, a Ravensbourne man brought me two kowhai twigs bearing three darkbrown pearlike objects from his section for identification. These irregular wrinkled brown growths were 34cm long, and were growing on the largeleaved kowhai
Sophora tetraptera, which is a North Island species often cultivated in the South. I was also recently shown a local species, S. microphylla, covered with dozens of these growths.
These were distortions of the kowhai’s seed pods caused by the rust fungus Uromyces
edwardsiae Cunningham. This rust fungus has been found only on kowhai trees (although the species Sophora microphylla, S. prostrata and S. tetraptera are all attacked by this fungus). It differs from many rust fungi by apparently lacking alternative hosts.
A Portobello property currently has the largest number of these distorted seed pods on kowhai trees the owners have ever seen. This may have been caused partly by stress on the kowhai from a
Pittosporum tenuifolium tree growing rapidly beside it, plus weeks of unusually wet, humid weather last month. The rust fungus, however, will have been long established under the bark of the infected kowhai.
Kowhai trees are damaged by the rust fungus in two ways. The fungus can attack living stems and twigs, causing them to become greatly thickened and foreshortened, with dense masses of twisting branches. These are termed ‘‘witches’ brooms’’, a name that dates from medieval times when superstitious people believed that witches caused the distortions.
When the fungus attacks seed pods, they soon become unrecognisable and eventually resemble wrinkled brown pears with a darkbrown powdery coat.
This powder is composed of large numbers of fungal spores, called teliospores, which are produced in special microscopic cavities on the ‘‘pear’’ surface. The spores are dispersed by the wind, ready to infect other kowhai trees.