SA Graan September 2015

FOCUS

Integrated pest control

Special

September 2015

Host plant resistance to wheat rusts:

Cost saving solution reducing

fungicide applications

here are three types of rust that infect wheat, and they are

stripe, leaf and stem rust (

Photo 1a - Photo 1c

). The rusts

are obligate parasites and cause damage by increasing tran-

spiration and respiration whilst decreasing photosynthesis.

Wheat rusts, like most fungi, require free water or moisture in order

for the spores to germinate and cause infection. The incidences of

the three rusts are dependent on climatic conditions and can vary

between seasons. Currently, stripe rust is the most problematic in

the summer rainfall area, whilst all three rusts affect wheat produc-

tion in the winter rainfall area.

South African wheat producers currently rely predominantly on the

use of fungicides to control wheat rusts. Fungicide applications may

be required more than once in a season depending on the severity

of rust infections.

Besides the negative implications that fungicides have on the en-

vironment, a reduction in fungicide usage will allow producers to

reduce their input costs resulting in higher profits. The constant use

of fungicides may potentially cause rust pathogens to develop resist-

ance to fungicides in the future.

The use of genetic resistance is an alternative method for rust con-

trol and is overall beneficial to both the environment and producers.

Understanding host plant resistance

There are two modes of resistance that can be categorised as

race-specific resistance and adult plant resistance. Race-specific re-

sistance is also referred to as seedling resistance and this type of

resistance is generally effective at all plant growth stages.

Resistance is conferred by major genes that offer complete resist-

ance to specific rust races and if not deployed strategically, it can

result in the gene being effective for a limited time only, due to the

formation of new rust races (

Figure 1

New rust races may occur through mutation or introduction from

a different area or country and can result in resistant cultivars be-

coming susceptible. Adult plant resistance offers partial resistance

against most races and functions mainly at the adult stage.

Seedlings are therefore still susceptible to rust infections. This type

of resistance is based on complex systems resulting in durable re-

sistance that is much longer lasting.

Breeding for rust resistance

A strategy that is currently used by many breeding programmes is to

target the deployment of adult plant resistance in combination with

race-specific genes in an endeavour to secure long lasting durable

resistance against wheat rusts.

Practically this can be challenging as some rust resistance genes

are sometimes located near/close to yield and quality genes, thus

negatively affecting yield and quality. This is further complicated by

KRISHNA NAICKER,

ARC-Small Grain Institute, Bethlehem

Infected wheat plants exhibiting symptoms of (1a) stripe rust,

(1b) leaf rust and (1c) stem rust.