PCC MarchApril Final Draft

PYTHIUM DISEASE CATEGORY
Seed decay
Pre-emergent
damping-off
Post-emergent
damping-off
Root and crown rot of
young plants
Root rot of mature
plants
Foliar blight
Vegetable soft rot
TARGETED PLANT TISSUE
Seed planted in the ground is invaded
and rotted before the seed germinates.
Seed germinates; root and shoot are
infected and killed before seedling
shoot emerges above the ground.
Seed germinates; root and shoot are
infected but seedling shoot is able to
emerge above ground before collapsing
and dying.
Young plants are healthy while
germinating and emerging; after
plant establishment, roots or
crowns become infected.
Roots of mature plants can become
infected later in crop development. This
results in “root pruning” and subsequent
reduction in growth, vigor, and yield.
Above ground leaves, stems, shoots
become diseased when the pathogen
is splashed up onto foliage.
Fleshy vegetables such as cucurbit
fruits, sweet potato roots, and
potato tubers develop soft decays.
Table 1. Categories of Pythium diseases of row crops
Continued from Page 12
of the roots and shoots that just grew out of the seed. This early
disease stage is often called damping-off. Damping-off is further
divided into two phases. If the newly germinated seedling is
infected so early and so severely that it dies before being able to
break through the soil surface, this situation is called pre-emergent
damping-off. However, post-emergent damping-off occurs
if the diseased seedling is strong enough to emerge above the soil
surface, only to succumb and collapse shortly afterwards. Collectively,
seed decay, pre-emergent damping-off, and post-emergent
damping-off can result in loss of plants very early in the production
cycle, causing stand loss in the field.
Healthy seedlings that escape death at the seed and newly germinated
stages remain vulnerable to this pathogen; established
seedlings can still be infected and become stunted and die due to
diseased roots and crowns. Older, established plants have escaped
the damping-off phase that kills seedlings but can be subject to
infections that prune back the roots, leading to reduced plant
vigor and yield. For example, Pythium can cause late infections
in cauliflower and result in weakened roots and poorly yielding
plants. This soilborne pathogen can even cause a foliar blight of
leaves and shoots, though this type of disease is not very common.
Bits of soil carrying Pythium can be splashed or moved
up onto foliage and cause blights on crops such as spinach and
bean. Finally, the fleshy parts of some vegetable crops are subject
to Pythium pathogens. If in contact with infested soil, cucurbit
14 Progressive Crop Consultant March / April 2021