SA Graan February 2019

Environment

From the above it is clear that

S. sclerotio-

rum

is highly dependent on its environment

– both weather and agronomic conditions

– for disease initiation, development and

survival. Cool and wet conditions favour

disease development. A daily plant surface

moisture of 12 hours to 16 hours or a con-

tinuous plant surface moisture of 42 hours

to 72 hours together with temperatures

ranging from 10˚C to 25˚C are required for

disease development.

The disease is more prevalent in fields were

crops have a dense canopy and air circula-

tion is limited, as a result creating a favoura-

ble micro-climate for disease development.

This is directly related to the population

density and row spacing selected at plant-

ing, as well as the selection of cultivars prior

to planting, as cultivars vary in their physio-

logical structure, i.e. determinate or indeter-

minate. Thus, increasing inter and intra-row

spacing or reducing plant populations per

hectare will increase air circulation which

can reduce the risk of Sclerotinia infection.

Hosts and tillage practices

This pathogen has a complex life cycle

and interaction with its hosts and environ-

ment, which makes the management of this

disease intricate. One of the first steps to

managing the disease is to purchase dis-

ease free certified seed. The South African

National Seed Organisation (SANSOR) uses

methodologies prescribed by the Interna-

tional Seed Testing Association (ISTA) for

evaluating registered seed.

The current limitation from ISTA of scle-

rotia in soybean seed is 0,2% in a 1 000 g

sample. This is required prior to the issuing

of a final seed lot certificate. The virulence

of the sclerotia is not tested during this pe-

riod. However, legislation states no more

than 4% sclerotia may be present in canola,

soybean and sunflower seed. Therefore, the

SANSOR regulations are much stricter, only

allowing 20 g of sclerotia per 10 kg, in con-

trast with the 400 g of sclerotia per 10 kg.

Seed treatments have been said to re-

duce the germination of the sclerotia pre-

sent. Preliminary tests at the University of

the Free State (UFS) of sclerotia, found in

treated sunflower seed, have indicated that

sclerotia may still germinate, although this

requires further elucidation.

Weeds are also known as alternative hosts

and thus it is crucial to reduce the quantity

of weeds present within and surrounding

host crop fields. This will reduce the build-

up of the fungus, either in the form of myce-

lium or sclerotia, in the immediate vicinity of

the host crop. Crop rotations with non-host

crops can reduce Sclerotinia inoculum lev-

els. However, the survival time of sclerotia

in soil as well as the extensive host range of

the pathogen should be kept in mind.

There are contradictory reports on the

effectiveness of tillage practices on control-

ling Sclerotinia diseases, due to the relation-

ship with sclerotial survival. Reduced tillage

promotes surface sclerotia, and reduces the

production of apothecia, while deep tillage

can promote re-emergence of sub-soil in-

oculum and sclerotia to the upper 5 cm of

the soil surface, which is the pathogen’s ac-

tive zone.

Burning crop residues is a common practice

to reduce pest populations. There is limited

information available on the effectiveness

of burning on sclerotia or mycelium survival

and viability on stubble. Reports state that

the amount of stubble, sclerotial size as well

as temperature and duration of the fire can

result in variable results of the sclerotia ger-

mination, and ultimately yields burning as

an ineffective method of control.

No resistant cultivars

Currently, in a worldwide context, there are

no commercially available resistant sun-

flower or soybean cultivars. However, the

manner in which soybean and sunflower

cultivars differ in their response to the

pathogen under disease-favourable condi-

tions differs and selection of more ‘tolerant’

cultivars can reduce the risk of infection,

yield losses and inoculum buildup in fields.

The response of soybean and sunflower cul-

tivars to disease potential can be quantified

using regression analysis. This analysis can

determine the type of response as well as

the relationship observed between Sclero-

tinia stem and head rot within a cultivar to

changing disease potentials. Disease poten-

tial is defined as the mean disease severity

over all cultivars of the respective crop un-

der specific environmental conditions.

Three response types can be observed be-

tween Sclerotinia incidence in a cultivar and

Sclerotinia potential:

Cultivar tolerance to increasing disease

potential.

Cultivar intolerance to increasing dis-

ease potential.

Cultivars having a linear relationship

with increasing disease potential.

This regression methodology can be an

effective and accurate tool to quantify the

response of cultivars to different disease

potentials and subsequently help the selec-

tion process of cultivars for a specific dis-

ease potential.

Manag ment of Sclerotinia

head and stem rot

ON FARM LEVEL

Integrated pest control

Februarie 2019

3a: Apothecia of

Sclerotinia sclerotio-

rum

and the innocent

bird’s nest fungus

(

Nidulariaceae

spp.).

3b: Commonly

misidentified as

apothecia.