[Illustration: Septoria canker on young cottonwood saplings.]
Septoria canker is caused by the fungus _Septoria musiva_. Although this
is a disease of poplars, native poplar species are not severely
attacked. However, this is an important problem wherever hybrid or
introduced poplars are grown. With the ever-increasing emphasis on
poplar planting, this will probably become a much more important problem
in the near future.
Young stem cankers usually develop around openings such as wounds,
lenticels, or leaf scars, appearing first as sunken, dark areas of the
bark. The infected area later becomes more depressed and darker as
tissue dies, and often a black margin will be formed around the canker.
Small, pink, hair-like spore tendrils are produced by the fungus around
the canker margin, especially during moist weather. These tendrils
contain spores which can cause further infection, and arise from small
dark fungal fruiting structures called pycnidia. This cankered area is
often invaded by insects and other fungi and is also a weakened area at
which wind breakage may occur. The fungus also causes a gray to black
leaf spot, which usually has a light colored center. These spots may
coalesce on a severely infected leaf and involve the entire leaf
surface. This leaf spot in itself causes slight damage, but acts as a
source of fungus spores which can cause stem cankers, and thus is
important in the overall consideration of the disease.
[Illustration: Defoliation in cottonwood caused by Septoria
leafspot.]
A control for this disease is to use native poplars or resistant hybrids
wherever possible. Wider spacing in plantations may also reduce the
humidity, decreasing the amount of infection.