SA Graan September 2017

September 2017

Sclerotinia stem rot on soybean

in South Africa

clerotinia

sclerotiorum

is a fungus

that feeds on soybean tissue, caus-

ing stem rot of soybean. This fun-

gus is found all over the world and

it has a wide range of host species including

important South African crops like soybean,

sunflower and canola.

The fungus thrives in wet and cold condi-

tions and can result in extreme yield losses

under favourable environmental conditions.

The stem rot starts as a white mould grow-

ing on the surface of infected plants, as

seen in

Photo 1

and

Photo 2

Water-soaked lesions then follow and vas-

cular tissue is replaced with fungal survival

structures called sclerotia (

Photo 3

), caus-

ing wilting and death of plant cells. Sclerotia

can also be found in soybean pods, as seen

Photo 4

, possibly reducing the quality

of seeds.

The cell death is known to be due to toxic

substances, the main virulence factor being

oxalic acid, which is secreted by the fungus.

However, the mechanisms involved in this

process are still unclear.

Seed testing

Losses caused by stem rot are mainly due

to reduced seed quality and lower yields.

Seed testing in terms of health, vigour and

germination is thus important to assess

quality of seeds marketed to customers.

Seed health is important as pathogens as-

sociated with soybean seeds can cause fast-

er deterioration of seeds in storage, a rise in

soil- and seed-borne inoculum, a decrease

in seedling emergence or germination of

seeds, or they could cause disease symp-

toms on mature plants, e.g. blights and leaf

spots. Seed health testing will provide in-

sights into which fungi are present in South

African soybean seed, and whether some of

these fungi are pathogenic on soybean.

The seed health of 29 different soybean

cultivars (

Table 1

) was determined using

a traditional incubation method. Possibly

pathogenic fungal species isolated included

Bipolaris, Chaetomium, Macrophomina

and

Fusarium

species. Results from seed health

testing are presented in

Graph 1

Macropho-

mina phaseolina

could possibly cause char-

coal rot on soybean roots and stems.

The seed health was calculated by dividing

the number of fungal isolates per cultivar

by the 400 seeds tested and converted to a

percentage. From the results of seed health

testing, growers can choose the optimal cul-

tivars for planting in each region, based on

amounts of pathogenic microflora present

in seed. This could prevent yield losses due

to reduced seed-borne inoculum levels.

Germination tests are used to assess the

quality of seeds, which has a direct influ-

ence on the productivity of crops. Seeds

with lower vigour and viability usually have

lower germination rates. Accelerated age-

ing is a vigour test commonly used for soy-

bean where seeds are rapidly aged under

conditions of high temperature and humid-

ity. This test estimates the storage potential

and field emergence of soybean.

In this study the normal germination per-

centages and accelerated ageing germina-

tion percentages of 29 soybean cultivars

planted in South Africa were determined.

From the results, it was found that cultivar

LS 6161 R had the best normal germination,

with a percentage of 96%. This cultivar’s ac-

celerated ageing percentage was also very

good at 86%.

PAN 1664 R was the cultivar with the low-

est germination percentage of only 53%,

as well as being the cultivar with the lowest

accelerated ageing germination percentage

of 34,5%.

Cultivars NS 5909 R and LS 6466 R showed

accelerated ageing germination percent-

ages higher than the normal germination

percentages, demonstrating that these

cultivars have good vigour, and should still

have relatively good germination at high

temperatures and relative humidity.

The highest accelerated ageing germination

percentage of 89% was recorded for culti-

vars PAN 1614 R and PAN 1729 R.

Resistance screening

against

S. sclerotiorum

Since stem rot caused by

S. sclerotiorum

can result in extensive damage, many con-

trol measures have been tested, includ-

ing partial host resistance, but these have

proven to be fairly ineffective. Due to the

ineffectiveness of management practices

like chemical control, biological control and

cultural practices, the identification and/or

development of cultivars that confer resist-

ance to the pathogen could provide more

effective disease management options.

FOCUS

Integrated pest control

Special

LIEZL VAN DER HOVEN

and

PROF JACQUIE VAN DER WAALS,

Department of Plant and Soil Sciences, University of Pretoria and

DR MARIETTE TRUTER

, ARC-Vegetable and Ornamental Plants, Roodeplaat

CULTIVAR NUMBER CULTIVAR NAME CULTIVAR NUMBER CULTIVAR NAME CULTIVAR NUMBER CULTIVAR NAME

LS 6240 R

PAN 1664 R

LS 6261 R

LS 6444 R

DM 5953 RSF

DM 6.2i RR

PAN 1454 R

LS 6453 R

LS 6164 R

LS 6146 R

PAN 1521 R

LS 6161 R

PHB 94 Y 80 R

PAN 1500 R

PAN 1614 R

LS 6248 R

NS 5909 R

NS 6448 R

NS 5009 R

PAN 1513 R

DM 6.8i RR

DM 5.1i RR

LS 6466 R

NS 7211 R

PHB 95 Y 20 R

PAN 1666 R

PAN 1729 R

PAN 1583 R

PAN 1623 R

TABLE 1: SOYBEAN CULTIVARS USED IN SEED HEALTH TESTING.

De Beer and De Klerk, 2015