SA Grain May 2017

Alternaria leaf blight

and seed infection:

A common disease causing major damage

lternaria leaf blight is caused by

Alternaria alternata

, which is the

predominant

Alternaria

spp. of

sunflower responsible for signifi-

cant yield losses in South Africa. A recent

five year survey carried out by ARC-Grain

Crops has shown the disease to occur in all

major sunflower production areas viz. Lim-

popo, Mpumalanga, North West Province

and the Free State.

Alternaria leaf blight is currently one of

the major potential disease threats to sun-

flower production in South Africa. Fortu-

nately, there is a collaborative research

programme between the University of Pre-

toria and ARC-Grain Crops to improve the

understanding of the causal organism, the

epidemiology of the Alternaria leaf blight

and to attempt to develop an integrated

control system based on fungicide sprays.

Recent studies by the University of Preto-

ria have indicated that

Alternaria alternata

is seed-borne (

Photo 1

) and is able to infect

seedlings from a very young age. These in-

fections take place during head formation.

Recent studies carried out by ARC-Grain

Crops and the University of Munster, Ger-

many, have shown these infected harvested

seed to be contaminated by two second-

ary metabolites (mycotoxins) namely tenu-

azonic acid and tentoxin, that were most fre-

quently found in the shelled and unshelled

sunflower seed samples. Tenuazonic acid

has been found to have acute toxic effects

on various mammals.

Epidemiology of

the disease

Alternaria

spp. overwinter on diseased

stalks in the field and has been reported to

be seed-borne. The sunflower seeds com-

monly distribute the primary inoculum. The

pathogen may contaminate the seed sur-

face or the seeds may be infected systemi-

cally from the mother plant.

The husk of the seeds is infected before the

infection spreads to the embryo of the seed.

During germination, the pathogen is trans-

mitted to the cotyledon or the seed coat.

The seed is infected and during germina-

tion, the pathogen is carried either passively

on the cotyledons, or on the seed coat. Dur-

ing systemic infections, the fungus grows

into the ovule and later develops from the

seed into the seedling.

The pathogen can be carried either pas-

sively or actively to their host. Air currents

and rain splashes aid in the dispersal of the

conidia to young florets of healthy heads.

The sunflower crop may be infected from

the outside by conidia being disseminated

by wind to young florets of healthy heads.

The epidemic of the Alternaria pathogen

commonly starts when the plants start flow-

ering. The epidemic reaches the maximum

intensity when plant senescence occur. The

factors that contribute to epidemics of Alter-

naria leaf blight include an increase in inoc-

ulum, environmental conditions that favour

an elevated level of spore deposition, and

predisposition to wounds.

Alternaria leaf blight is usually favoured by

temperatures of 25°C to 30°C and twelve

hours of leaf wetness.

The disease is initiated when spores land

on leaves or stems, germinate in the pres-

ence of moisture and directly penetrate and

infect the leaves.

When the environmental conditions are fa-

vourable, the Alternaria condiums are able

to germinate within three hours.

The conidia are able to produce multiple

germ tubes; these tubes penetrate the leaf

cells directly through the cell walls or sto-

mata.

PROF BRADLEY FLETT,

ARC-Grain Crops, Potchefstroom

1: Infected sunflower kernels showing

growth of

Alternaria alternata

on a petri

dish.

Photo: Godfrey Kgatle

2: Leaf symptoms showing circular, dark

brown lesions with concentric rings.

Photo: Moses Ramusi

ON FARM LEVEL

Integrated pest control

Potential disease threats / Sunflower production

May 2017