SA Grain May 2017

Alternaria leaf spot:

Assessment of disease incidence in areas of concern

his season’s wetter climatic conditions are ideal for sun-

flower diseases. Grain SA received several complaints from

producers pertaining to this. Concerns were specifically high

from producers in the Hoopstad/Wesselsbron region where

various diseases were a problem, with the highest incidence being

that of Alternaria leaf spot, a severe problem in the current season.

Grain SA together with two researchers from the University of

Pretoria, Prof Terry Aveling and Mr Godfrey Kgatle (PhD candidate),

visited three different localities in this area on 27 February to ob-

serve the severity of the problem. Kgatle was contacted as he is in

the process of completing his PhD studies on this disease’s occur-

rence around the country.

The researchers collected samples based on noticeable symptoms

from the field. Alternaria leaf blight (

Alternaria helianthi

and

Alter-

naria alternata

) and Phoma black stem (

Phoma macdonaldii Boer-

ema

) was found on the sampled leaves, stem, petioles and debris.

The researchers stated that there was a lot of debris in the field

that contained the above-mentioned pathogens and that there was

a lack of adequate crop rotation from the previous season.

They expect that these diseases would have been worse this sea-

son due to inoculum build-up and suitable weather conditions (wet

conditions accompanied by mean temperatures of between 25°C

and 30°C).

See the article on page 47 for more information on Alternaria

1: Producers present at the field survey with Prof Terry Aveling (centre)

and Godfrey Kgatle (far right).

2: Alternaria leaf spot (a) and Phoma black stem disease (b) symptoms

from samples collected during field visits.

3: This photo shows the weakening effect of Phoma on the leaf petiole.

4: Underfilled sunflower grains due to Alternaria defoliation and early

senescence/maturity due to the disease.

5: Stem lodging due to the stem weakening effect of Phoma.

ON FARM LEVEL

Integrated pest control

Wetter climatic conditions / Sunflower diseases

PETRU FOURIE

and

WANDILE NGCAMPHALALA,

Grain Research and Policy Centre, Grain SA