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PYTHIUM INCREASES STUNTING IN CANKER AFFECTED CAULIFLOWER
B.H. Hall and L. Deland
South Australian Research & Development Institute, Adelaide, SA; Barbara.hall@sa.gov.au
ABSTRACT. Stem canker of Brassica is a disease complex of several fungi causing symptoms that range from superficial
scurfing, russetting and discrete lesions on stems to complete stem rot, girdling and plant collapse. The primary pathogens
involved are Leptosphaeria maculans and Rhizoctonia solani AG 2.1, 2.2 and 4. Pythium irregulare and P. ultimum were
also recovered from infected plants but initial investigations showed they did not cause canker. Cauliflower seedlings grown
in inoculated soil showed that both Pythium species reduced canker severity caused by Rhizoctonia or Leptosphaeria.
However they also significantly reduced plant growth and head weight.
INTRODUCTION
Pythium spp. are common soilborne organisms that cause
root rotting and seedling damping off in many plants,
including brassica (1). Previous work on stem canker of
brassica showed P. ultimum and P. irregulare were isolated
from affected plants, along with the primary pathogens
Rhizoctonia sp. and Leptosphaeria maculans (2). However
neither of the Pythium species produced canker symptoms
and their role in the disease had not been elucidated.
This report outlines the results of growthroom experiments
investigating the effect on growth of cauliflower seedlings
planted into soil inoculated with Leptosphaeria and/or
Rhizoctonia with and without Pythium.
MATERIALS AND METHODS
Experiment 1: 200g of sterile pre moistened millet was
inoculated with P. irregulare, incubated for 7-10 days then
mixed with 25Kg sterile cocopeat and placed into 200mm
diameter pots. 20 replicate six week old cauliflower cv.
Chaser were planted into either the Pythium cocopeat or uninoculated
cocopeat and inoculated with Rhizoctonia AG2.1
by placing two 7mm plugs of infected agar next to the stem
of each plant and covering with cocopeat. At 13 weeks after
planting the disease severity was rated as 0, 25 (25% of stem with canker), 75 (plant
wilting) or 100 (plant death). Plants were harvested at 15
weeks and heads weighed.
Experiment 2: 12 Kg of sterile cocopeat was mixed with
100ml of mycelia slurry of R. solani AG2.1 and AG4 and
incubated for 7 days. 50g of millet inoculated with P.
ultimum was mixed with 6Kg of the Rhizoctonia inoculated
soil. 20 replicate MK6 pots (~6 cm square) were filled with
either the Rhizoctonia inoculated soil, the mixed inoculum
soil, or sterile cocopeat mixed with sterile millet used as a
control.
Three week old cauliflower seedlings cv. Discovery were
washed clean of soil and dipped in a suspension of ~10 6 L.
maculans spores/ml for 2 mins before planting into the
inoculated soil. Washed seedlings dipped in sterile water
were planted into the control soil. The length of the first fully
expanded leaf from leaf apex to leaf base was measured ten
medium or large and the disease severity rated as previously
described.
RESULTS
Experiment 1: Plants grown in soil infected with
Rhizoctonia and P. irregulare had significantly lower canker
severity, head weight and fresh weight than plants grown in
Rhizoctonia infected soil without Pythium (Table 1).
Table 1. Plant growth and canker severity of cauliflower cv.
Chaser grown in soil inoculated with Rhizoctonia solani
AG2.1 with and without Pythium irregulare.
Canker severity
(13 weeks)
Head
weight (g)
Fresh
weight (g)
-Pythium 39% 189 450
+ Pythium 25% 92 399
P