PCC MarchApril Final Draft

A REVIEW OF PYTHIUM
DISEASES IN ROW CROPS
Soilborne Organism
is Known for Causing
Seedling Diseases and
Other Issues
By STEVEN KOIKE | TriCal Diagnostics
Large, established cauliflower plants can still become
infected with Pythium (left), resulting in loss of roots,
severe stunting and yield loss (all photos courtesy S.
Koike.)
It is highly likely that growers, PCAs
and other field professionals are
familiar with the word “Pythium”.
Pythium is the name of a soilborne,
fungus-like organism that is notorious
for primarily causing seedling diseases.
Pythium is notable because many row
crops are susceptible to it, the pathogen
is very widely distributed and occurs in
most cropped ground, and despite the
use of IPM tools and strategies, Pythium
problems can still show up in row
crop production systems.
What is Pythium?
Pythium is a fungus-like organism. Previously
considered to be a true fungus,
molecular studies in recent years indicate
that Pythium—as well as closely related
organisms like Phytophthora and
downy mildew—is more closely related
to brown algae and diatoms. Formally,
therefore, Pythium species are no longer
part of the fungal taxonomic group but
are classified in the kingdom Chromista,
or Stramenopila. The Pythium
genus contains over 200 species, most
of which are not plant pathogens. There
are Pythium species that are pathogens
of animals (some of which can infect
humans), and many species are saprophytes
and only grow on dead
and decaying organic material.
Pythium species are mostly
found in soil environments
but are also present in aquatic
habitats.
Plant pathogenic Pythium
species are well equipped to
cause problems on row crops.
Most of these species form
resilient, thick-walled sexual
spores (oospores) that can
withstand periods of unfavorable
dry and warm conditions.
These structures enable
Pythium to persist in the soil for a long
time. When favorable soil conditions
are present, mostly in the form of abundant
soil water, these Pythium organisms
either produce hyphae that grow
toward the roots or swimming spores
(zoospores) that move through the soil
water in search of susceptible plant
tissues. Another feature that makes
Pythium problematic for growers is the
extremely fast growth rate of these organisms.
Given suitable soil conditions,
Pythium pathogens can rapidly grow
from seed-to-seed, seedling-to-seedling
The primary symptom of Pythium diseases is the dark
discoloration and decay of roots, pictured here on
lettuce.
and root-to-root.
Diverse Pythium Diseases
In contrast to many plant pathogens,
Pythium causes several different types
of problems on crops (Table 1, see page
14). First, Pythium is a seed pathogen.
Once placed in the ground, seed can
be exposed to Pythium that is residing
in the soil. If conditions are favorable
for the pathogen, Pythium can invade
and colonize the seed, causing it to
rot before it can germinate. If the seed
germinates, Pythium can cause a decay
Continued on Page 14
12 Progressive Crop Consultant March / April 2021