SA Graan Desember 2017

December 2017

Unnecessary stressors on the crop may

increase the potential for Diplodia stalk

rot, which indirectly in the long term may

increase inoculum levels on the land and

under conditions favourable for Diplodia ear

rots an epidemic may occur.

Crop rotation

Crop rotations reduce Diplodia ear rots by

reducing inoculum levels in two ways. First-

ly, a non-host for the fungus will not allow

the fungus to persist for the season where

maize is not grown. Secondly, a greater pe-

riod (a season or two) between maize crops

allows for a natural breakdown of maize

stubble, which again reduces the survival of

the fungus.

Leguminous crops, such as soybeans, dry

beans, groundnuts and cowpeas are very

good rotation crops. Other rotation crops

that reduce Diplodia ear rots, are wheat and

oats. Sunflowers do not significantly reduce

Diplodia ear rots under experimental condi-

tions. The reason for this has not yet been

found.

Early harvesting

Early harvesting will reduce Diplodia ear

rots as it reduces the time available for the

fungus to grow on the ear. Late or winter

rains keep ears wet and increase the chance

for fungal growth. In certain cases, it would

pay to harvest early at higher moisture

levels and artificially dry grain. This is possi-

bly why Diplodia ear rot is not a major prob-

lem in the USA where maize is harvested

early and dried artificially.

Hybrid resistance

Selection of cultivars is very important in

the control of Diplodia ear and stalk rots.

In general, a resistant hybrid will always

have less Diplodia ear rot than susceptible

hybrids relative to prevailing conditions.

This effect is an interaction between the

available inoculum, the host and prevailing

weather conditions.

However, there are many maize hybrids

that react consistently over all localities,

but there are some that do not. In a project

funded by the Maize Trust, the local South

African National Cultivar Trial entries of the

ARC are used to screen maize hybrids annu-

ally for resistance/susceptibility to Diplodia

ear rots under various weather and inocu-

lum conditions. Diplodia inoculum is also

supplied to companies for screening.

Resistance to Diplodia stalk rot is difficult to

quantify as plant standability or resistance

to lodging, does not necessarily mean the

stalk is not infected with Diplodia stalk rot.

It means that even though the stalk may be

infected with Diplodia stalk rot, it will not

lodge. Resistance therefore is not against

the fungus, but by improving stalk rind

thickness.

A thicker rind may still have Diplodia stalk

rot which results in the breakdown of the

stalk pith tissue within the rind. Thicker

rinds may in the long term have the unin-

tended effect of actually increasing Diplodia

inoculum as it is more resistant to decom-

position and may improve survival of the

inoculum which from extensive studies has

been shown to survive successfully in intact

maize residues, particularly those retained

on the soil surface. It is therefore important

that Diplodia stalk rot resistance is seen in

the correct context.

Diplodiosis and its

associated toxins

Diplodiosis, a nervous disorder of cattle and

sheep, results from the ingestion of ears

infected by Diplodia. Cases of diplodiosis

occur from six days to two weeks after the

animals are placed on fields with infected

maize cobs.

The disease is characterised by reluctance

of the animals to move, a wide-based

stance, inco-ordination, tremors, paralysis

and death. The disease also causes abnor-

mal foetal development and foetal death.

Field outbreaks of diplodiosis in southern

Africa are favoured by late, heavy rains and

occur during the late winter months (July to

September). The practice of using harvest-

ed maize fields for winter grazing is a ma-

jor contributing factor to outbreaks of this

mycotoxicosis. In addition to diplodiatoxin,

new metabolites, namely dipmatol, diplo-

nine and chaetoglobosins K and L, have re-

cently been isolated from Diplodia infected

crops.

To date, none of these pure metabolites

have been administered to ruminants in

order to reproduce the disease. Laboratory

analytical test methods that quantify and

establish the presence and distribution of

these toxins in infected maize commodities

are also lacking.

The future

In a current Maize Trust funded project,

the University of Pretoria (Departments

of Chemistry and Veterinary Science), the

Department of Biotechnology and Food

Technology (TUT), South African Grain

Laboratories (SAGL) and ARC-Grain Crops

are currently collaborating in producing the

various Diplodia mycotoxins in sufficient

quantities to develop an analytical test for

the detection of the metabolites in infected

maize as well as to confirm Diplodiosis of

these metabolites in target animals.

Who will benefit from

this research?

Little research has been done internation-

ally and this technology will give South

African maize producers, livestock produc-

ers, regulatory authorities and final consu-

mers a distinct advantage. The availability

of an analytical test method will ensure that

maize products supplied to the wide range

of consumers are free of the mycotoxins

rendering them safe.

Animal feed samples can be routinely test-

ed for the presence of Diplodia mycotoxins

and furthermore cases of diplodiosis can be

confirmed or disproved enabling interven-

tion methods. The technology will benefit

the South African and international research

community, enabling new avenues of re-

search that will give practical

solutions to all beneficiaries

involved.

1: A maize ear overgrown with white

mycelial growth.

2: A cross section of an infected maize ear,

showing black spore-producing bodies at the

kernel bases.

3: Lodging of maize plants due to Diplodia

stalk rot.

Photo: Prof Bradley Flett