SA Graan September 2017

September 2017

Procedures in the diagnosis of plant diseases:

The path to effective control measures

lant health problems usually arise from a variety of abiotic

(environmental factors such as cold, heat, salinity, drought)

and biotic (plant pathogens causing diseases and insects

damaging the plant) stress factors.

Unfortunately, the visual symptoms of these health problems are

often similar. Both abiotic and biotic stresses cause major crop

damage and result in significant crop losses annually. In order to ef-

fectively manage the impact of a certain disease, an accurate dis-

ease diagnosis is mandatory so that the impact of the disease can be

managed effectively and timeously.

Studies have shown that failure to adequately manage a disease can

often be traced back to a failure to correctly diagnose the problem,

resulting in wasteful expenditure of chemicals and other control

methods.

It is important to note that plants will invariably be affected by

biotic and abiotic factors, often in combination. To break the disease

cycle, producers must incorporate management strategies that will

effectively reduce or control the disease.

In order to incorporate such management practices, accurate plant

disease diagnosis plays a crucial role. Often, more than one strategy

is necessary for optimal disease management but also, producers

should consider any potential economic, environmental and social

impacts when implementing such management strategies.

During the development of a disease, three elements are required,

namely a pathogen (disease causing virus, bacteria or fungi), a sus-

ceptible host (the plant) and favourable environmental conditions

for disease development.

The combination of these three elements are called the disease tri-

angle. All three elements of the triangle must be present for the dis-

ease to develop. When one of the legs of the disease triangle are

unfavourable, the disease cycle can be broken and the disease will

not develop.

Abiotic factors

Abiotic disorders are usually caused by temperature, moisture, soil

pH, air quality, light regime and nutrition. If any of these factors

fluctuate from the optimum range for a given plant species, plant

growth might be adversely affected.

Often abiotic factors causing symptoms are diagnosed as a plant

disease. For example, sunburn of maize leaves (

Photo 1

) is often

confused with northern corn leaf blight caused by a fungus. Upon

misdiagnosis of the ‘disease’, the producer will then be advised to

apply a fungicide, which will be ineffective in solving the problem

and will cause the producer to incur unnecessary cost.

Abiotic disorders may also be caused by human interference,

such as incorrect pesticide and fertiliser applications causing plant

tissue to exhibit chlorosis and/or necrosis. A good approach to the

sound identification of a plant disease is to start off by deductively

eliminating any possible abiotic disorders and insect damage.

Symptoms caused by biotic factors

Insects

Many insects, including caterpillars and beetles, are relatively large

and easy to spot. The damage that they cause is often visible as

holes in the leaves (

Photo 2

) or fruit bodies of a plant, a frayed ap-

pearance of leaves chewed along leaf margins or deformed leaves.

Additional indications of the presence of insects include frass (dark

coloured insect excreta) and sooty mold (a black superficial growth

of a fungus on honeydew usually associated with sucking insects).

Viruses

Viral diseases can sometimes be difficult to identify because symp-

toms caused by viruses vary from plant to plant. The expression

of these symptoms can also be affected by the plant’s age and its

growing conditions.

An example of a viral disease is mosaic viruses. This virus is mostly

spread by sucking insects such as aphids and leafhoppers. Symp-

toms of this disease include leaves mottled with yellow, white, light

and dark green spots or streaks (

Photo 3

). Occasionally, the leaves

are reddish or purplish.

Infected plants grow poorly and may have other deformities or

wavy leaves. Leaf margins appear torn because of unequal growth

or feel rough because of swollen veins. Unfortunately, there are no

control procedures in place for plants already infected by mosaic

viruses. Infected plants must be removed and destroyed.

Preventative management practices will include the planting of re-

sistant genotypes and the control of insect vectors with their weed

hosts adjacent to fields.

Bacteria

The diagnosis and identification of bacterial diseases are done pri-

marily on the disease symptoms that can be seen in the constant

presence of large numbers of bacteria in an affected area.

For instance,

Erwinia

species usually cause stalk rot, crown rot,

stem rot, or fruit collapse. Typical symptoms include water soaked

lesions (

Photo 4

) exhuming a pungent odour. Upon microscopic ex-

amination, one will be able to see bacteria oozing out of an infected

plant part.

Management practices will include improved sanitation upon

handling of the crop and the application of copper-based chemicals.

Irrigation schedules can also be changed to the cooler afternoon

where possible, because these bacteria multiply rapidly in warm

and moist conditions.

Fungi

Fungi obtain their nutrients through absorption of organic matter

and are the cause of the majority of plant diseases. They are capable

of attacking all plant parts and show symptoms varying from root-,

stalk- and ear rots to foliar diseases.

Some fungi cause localised lesions while others spread system-

atically within the plant. A number of fungi that affect the above

FOCUS

Integrated pest control

Special

DRS ADRIAN ABRAHAMS

and

BELINDA JANSE VAN RENSBURG,

ARC-Grain Crops, Potchefstroom