Alan
McNab
Professor of Plant Pathology |
|
| PEA
DISEASES |
| |
Root
Rots are caused by several specific soil fungi. These fungi
cause conspicuously discolored and rotted areas on the main root and
at the base of the stem. It is difficult to distinguish between root
rots caused by these fungi. Several fungi may cause root rot on the
same planr. Specific symptoms, characteristic for some root rots,
are noted below. Diseases are named after the organism that causes
the rot.
These root-rot fungi persist in soil and in residue from diseased
plants. In addition, Ascochyta is seed-borne. Excessive soil moisture,
whether from heavy rainfall or from poor drainage, favors root rots.
Root rots are more likely to develop on weak, slow-growing plants
than on those that are vigorous or more rapid in growth.
Aphanomyces Root Rot
Fusarium Root Rot
Rhizoctonia Root Rot
Ascochyta Root Rot |
| |
Fusarium
Wilt and Near-Wilt are two major wilt diseases caused by different
races of the fungus Fusarium oxysporum f. sp. pisi. Symptoms and controls
for these two wilts differ, although they are caused by related fungi.
Fungi that cause both diseases can be introduced with the seed. Once
present, these fungi persist indefinately in soil. Fusarium wilt often
is detected by appearance of yellowing of lower leaves and stunting
of plants. Leaflet margins curl downward and inward. The stem may
be slightly swollen and brittle near the soil sile. Internal woody
stem tissue often is discolored, turning lemon brown to orange brown.
Externally, the root system appears healthy; however, secondary root
rots are likely to occur on plants wilted for long periods. Eventually,
wilted plants may die.
Near-wilt symptoms are similar to Fusarium wilt symptoms. however,
near-wilt plants die more slowly than do Fusarium wilt plants. Also,
internal woody stem tissue usually is brick-red in near-wilt plants.
This discoloration extends throughout the plant |
| |
Blights
of greatest signifigance are Ascochyta blight and bacterial
blight.
Ascochyta Blight
Bacterial Blight |
| |
Powdery
Mildew, caused by the fungus Erysiphe polygoni, is characterized
by a white powdery talcum-like mold on leaves, stems, and pods. Tiny
dark perithecia (fungus reproductive structures) may develop among
mold strands late in the season. When pods are affected, seeds may
be discolored gray or brown; the fungus can be seed-borne. Reproductive
spores formed on the white mold and in perithecia are spread by wind.
Germination of spores from the white mold is promoted by humid conditions
but not by wet conditions. Powdery mildew is more important in souther
states than in northern areas, presumably because fungus spores are
available continuously in southern areas. In northern areas, powdery
mildew is initiated by fungus structures introduced on seed or blown
by wind. |
| |
Viruses
case four important pea diseases. Each disease is caused by a different
virus and has its own symptoms. Viruses that cause these diseases
overwinter in certain plants (e.g., clovers, vetches, alfalfa) of
the legume family. Aphids acquire virus from infected plants and transmit
virus to other plants during their feeding activities.
Pea Enation Mosaic
Pea Stunt
Bean Yellow Mosaic
Pea Streak
|
|
