Vegetables Australia May-June_MR

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R&D | SOIL BORNE DISEASES |
In this edition of Vegetables Australia, researchers Dr Doris Blaesing and Dr Len Tesoriero discuss the Pythium
species that cause cavity spot and forking in carrots. This is one of several soil borne diseases under the
microscope in a vegetable levy-funded project that aims to assist growers in the management of these diseases.
INVESTIGATING CAVITY SPOT AND FORKING IN CARROTS
Two main Pythium species have been identified to cause
forking and cavity spot of carrots in Australia. In most cases,
P. sulcatum causes the symptoms while P. violae has only
been identified in South Australia to date. Further research is
happening to its spread.
P. sulcatum, the main pathogen causing cavity spot of
carrots in Australia, mostly affects the carrot family of plants.
It also causes severe root rot diseases of parsley and coriander.
P. violae is the main cause of cavity spot of carrots in most
other countries and has a much wider host range.
The primary sources of Pythium inoculum are dormant resting
spores formed during colonisation of plant tissue, and can
survive in the soil for several years.
Cavity spot caused by P. sulcatum is most severe in summer
and autumn harvested crops. In wet soils, this species also
produces mobile spores (zoospores), which are attracted
to roots where they encyst and cause infection. Although
zoospores only survive for a day or so, they can significantly
increase the population concentration of this pathogen, which
can lead to multiple infection sites on any one carrot.
P. violae does not produce mobile spores; it produces
spherical swellings that spread with irrigation water. Cavity spot
caused by this species is most severe in winter harvested crops.
CAVITY SPOT DEVELOPMENT AND MANAGEMENT
The main factors affecting cavity spot development are soil
temperature, soil pH and soil moisture. While temperatures
can be controlled to a degree via site selection and scheduling
planting times, other factors can be controlled by crop
management approaches.
Temperature: The prime growth temperatures for P. sulcatum
are minimum 2-3°C, optimum 20-28°C and maximum 36-37°C.
Temperatures of 30°C and above are lethal for P. violae.
Soil moisture: High soil moisture leads to greater incidence
and higher severity of Pythium infections. However, at the
critical crop growth stages, the threshold soil moisture and the
period required at that threshold to cause infection with both
species is still unknown for Australian production regions.
Variety selection: This can greatly help in minimising
the occurrence of cavity spot, as some varieties are more
susceptible than others. It is worth asking seed suppliers.
Crop protection options: Metalaxyl-M can reduce the
incidence and severity of cavity spot disease when applied at
or shortly after seeding. However, if it is used too frequently it
can lose its effectiveness because of an increase in its rate of
breakdown in soil. While metham sodium is used commercially
for carrot production to manage the disease, enhanced
breakdown with repeated use has been implicated and it has
failed to control cavity spot in trials in Western Australia.
Soil pH: In Western Australia, it has been shown that liming
soil to increase pH reduces the incidence and severity of cavity
spot. The recommended pH range is pH 6.5-7.5 with a target pH
of 7.2 or higher (measured in calcium chloride).
Nutrition: UK research found that increasing the level of
exchangeable calcium above 8 meq/100g soil decreased the
incidence of cavity spot. High inputs of available calcium
pre-planting (e.g. 15 t/ha of a product called Limex) also
decreased cavity spot incidence. In both cases, P. violae was
the target organism.
Rotation: Research has shown that rotation with broccoli,
lettuce or onions has produced promising results where the
primary pathogen was P. sulcatum. As P. violae can attack
broccoli and may exacerbate cavity spot, rotation with
onions, corn, potatoes or beans may be more beneficial.
Cover crops/biofumigation: Reports on the benefits of
cover crops and biofumigants vary. The conclusion is that
the effect of cover crops on P. sulcatum and P. violae is
currently not well enough understood to make general or
regional recommendations.
Other: Cultural practices that reduce the impact of root
diseases include crop hygiene, selection of planting date
and crop density, tillage approaches that ensure good soil
structure and drainage, crop residue management to foster
their breakdown, and timely harvest. Integrated crop protection
(ICP) strategies to reduce the likelihood of infection using
management practices listed above are recommended.
THE BOTTOM LINE
While some general rules apply, especially the need for managing
soil moisture, pH, soil calcium and crop maturity, carrot producers
should find their own optimum combination of additional
management strategies that fit their production systems and
growing conditions.
A substantial research effort has been made to predict Pythium
inoculum levels and disease risks in vegetable crops, including
carrots. Most research had a focus on identifying threshold
levels of inoculum rather than identifying conditions (e.g.
temperature, soil moisture, soil nutrient levels, levels of other
diseases or pests) that cause infections to occur in different
commercial production systems.
Researchers at the South Australian Research and Development
Institute (SARDI) are currently developing soil DNA tests for
detecting soil populations of P. sulcatum and P. violae (VG15009).
Once these have been developed and tested, the next step is to
understand the relationship between soil inoculum levels and
production factors, both environmental and cultural practices.
For more information, please contact Dr Doris Blaesing on 0438 546 487
or or Dr Len Tesoriero on 0447 623 487 or
A multifaceted approach to soil borne disease management has been
funded by Horticulture Innovation Australia Limited using the research
and development National Vegetable Levy and funds from the
Australian Government.
Project Number: VG15010
INFO
R&D
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Peter Anderson, Northern Agronomist
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Years of Growth
PROUD
AUSVEG
STRATEGIC
PARTNER
• Nutrigation • Foliar feeding
• Controlled release nutrition
Australia’s largest
supplier of the
highest quality
water soluble
potassium nitrate
Your Haifa Australia team
Trevor Dennis, Managing Director
0400 119 852
Peter Anderson,Northern Agronomist
0459 488 850
Jon Corona, Southern Agronomist
0408 568 605
Jason Teng, Customer Service/Logistics
0488 036 528
T: 03 9583 4691 E:
Years of Growth
PROUD
AUSVEG
STRATEGIC
P RTNER
• Nutrigation • Foliar feeding
• Controlled release nutrition
ustralia’s largest
supplier of the
highest quality
ater solu le
potas iu nitrate
Your Haifa Australia team
Trevor Dennis, Man ging Director
04 0 119 852
Pet r Anderson, Northern Agron mist
0459 488 850
Jon Cor na, Southern Agron mist
0408 568 605
Jason Teng, Customer Service/Logist cs
0488 036 528
T: 03 9583 4691 E: a
Years of Growth
PROUD
AUSVEG
STRATEGIC
PARTNER
• Nutrigation • Foliar fe ding
• Controlled rel ase nutrit on
Pythium in carrots can cause the roots to fork or develop
black lesion or 'craters'. Image courtesy of RMCG.