Foliar Diseases/Conditions of Potato
In Michigan two diseases are most common on potato foliage. These are
late blight and early
blight. Below is a list of some common foliar diseases that may
be confused with late blight. Many of these organisms also can infect
tubers. For information on tuber diseases please visit the tuber
Blight of Potato
Late blight is caused by the fungus-like oomycete Phytophthora infestans.
This organism is generally treated as a fungus and can cause both foliar
and tuber infections. The pathogen is dispersed via wind-borne sporangia
which produce flagellated zoospores. These zoospores can infect leaf
tissue through either direct penetration of the epidermis or through
stomata. Once inside the plant the pathogen begins to kill the tissue,
causing the distinct foliar lesions. Control is accomplished by using
a protectant fungicide program.
Blight of Potato
Early blight is caused by the fungus Alternaria solani. Conidia
are approximately 200 µm long and have several transverse septa. The conidia
possess a tail or beak. The fungus overwinters in debris and is hardy
in Michigan. The lesions produced are often confused with late blight.
Early blight lesions show a general dry "bulls-eye" pattern
and do not usually spread very far and rarely affect petiole tissue.
In contrast, late blight lesions are appear watery and spread down the
petiole and the stem. A. solani can cause tuber rot but unlike
late blight, infected tubers are not generally susceptible to colonization
of secondary pathogens. Protectant fungicide programs used to control
late blight are generally effective at controlling early blight. A related
fungus Alternaria alternata produces similar lesions.
Mold of Potato
White mold, also called Sclerotinia stem rot, is caused by the fungus
Sclerotinia sclerotiorum (Lib.) de Bary. It is prevalent in
the Pacific Northwest but in Michigan it is of secondary importance
except in wet seasons or under excessive irrigation. The disease favors
very moist conditions and is especially common in fields with overhead-irrigation
such as by means of a center pivot. Agricultural practices that promote
extensive canopy growth and keep relative humidity and free moisture
in the crop canopy for extended periods of time and reduce wind movement,
favor disease development.
Botrytis Leaf Rot
Botrytis leaf rot is caused by the fungus Botrytis
sp. This pathogen also causes gray mold. The disease is usually the
result of excessive humidity and other stress and does not usually pose
a large threat to the crop. The fungus overwinters on plant debris.
Tuber infection can occur if the inoculum levels are high and the storage
facility is very humidity. Standard fungicide protection programs offer
control when conditions are not too wet.
Leaf Roll Virus
This virus is aphid vectored and tuber borne. Foliar symptoms include
upward rolling of the leaves and mild chlorosis. The virus damages phloem
tissue and can infect tuber tissue. The only control mechanisms are
to plant virus free seed and to control aphid populations in the field.
This virus causes leaf mottling and a reduction is plant size. Infections
can reduce tuber yield and quality. The virus is vectored in a similar
manner to that of leaf roll and can be controlled by using virus free
seed and controlling aphid populations.
The disease is caused by fungi in the genus Verticillium. The
fungus causes foliar wilting, chlorosis, and discoloration of the vascular
ring in both the stem and tuber. Tuber infections may be confused with
late blight. Verticillium overwinters in the soil and be can
be control by using clean seed, seed treatments, and rotations to reduce
soil inoculum levels.