Plant Disease

A plant disease is a disturbance brought about by a factor, which interferes with the manufacture, transport and utilization of energy sources or mineral nutrients and water in such a way that plant growth is negatively affected. Plant diseases are caused by pathogens and environmental factors. A plant pathogen is an organism which grows on a plant which provides growth factors. These organisms are generally not capable of producing their own food sources. The pathogenic organisms that cause disease are funguses, bacteria, mycoplasms, viruses and nematodes. Environmental factors which can cause plant diseases include temperature extremes, soil moisture extremes, and light extremes, lack of oxygen, pollutants and nutrient stress. This section deals with the pathogenic origins of plant diseases.

Click here to view a video that explains plant disease.

Fungal Disease

• Fungi cause fungal diseases.
• Fungi are plants that do not contain chlorophyll and thus they are not green and cannot convert sunlight energy to chemical energy.
• Fungi cannot produce carbohydrates for own usage.
• The vegetative parts of fungi consist of thin filaments known as hyphae, which masses together form a body or a mass known as a mycelium
• Fungi reproduce vegetative through the filamentous hyphae and sexually through spores.

Diagrammatic representation of the fungal filaments, known as hyphae (right) and a hyphal mass known as mycelium (left).

Fungi as a group is divided into two sub-groups, the slime moulds (lower fungi) and the true fungi (higher fungi). Disease- causing fungi are found in both these groups.

The Lower Fungi

Diseases are caused by three distinct groups of lower fungi. These diseases are discussed below:

Diseases caused by lower fungi without hyphae - are soil borne diseases that have a limited host range. The fungi rest and survive as resting spores in soil or in infected plants. They spread through movement of infected plant material, soil and water. Diseases of this nature are clubfoots in crucifers, powdery scab and wart disease in potato and maize brown leaf spot

Diseases caused by lower fungi with hyphae and zoospores – these fungi produce mycelium, form resting spores, produce living swimming spores (zoospores) which infect plants and spread through infected soil, infected plant material and through airborne sporangia. Examples of these types of fungal diseases are downy mildew and late blight.

Fungal diseases caused by lower fungi with hyphae but without zoospores - produce mycelium, produce air-borne thin walled spores and are generally weak parasites which grow on dead plant material. These fungi will attack live plant tissue only if the plant is under stress. Examples of these diseases are pumpkin fruit rot, vegetable soft rot, and bread mould and fruit rot.

Photo Description:

Examples of common fungal diseases of plants. (a) Rust on flax (Linum usitatissimum); (b) damping-off disease; (c) Microbotryum violaceum-infected Caryophyllaceae; (d) anthracnose of mango (Mangifera indica) leaves; (e) corn smut on a corn (Zea mays) ear; (f) anthracnose of mango fruit ((g) powdery mildew of papaya.

The Higher Fungi

Sooty moulds - causes a flat black coloured film on leaf surfaces which can be rubbed off, leaving a clean surface. It is common in warm, humid areas and is found on all crops. The organisms are not parasitic but use sugary excretions from aphids as food source. The primary damage caused is lowered photosynthetic rates.

Leaf curl – causes leaf blistering and swelling that lead to leaf curl. Blisters are hollow and are found mainly on stone fruits. The disease leads to leaf drop which could weaken trees. The fungus survives in the buds of trees.

Powdery mildew – A common disease, white to grey in colour, covering either the entire leave or part of it and usually on both sides of the leaf. It is found on young tissues and only grows on the plant surface. They are parasites and infestations are most severe under warm, dry weather conditions.

Foliar diseases – generally leaf-spots and blights but may also affect other plant parts. May survive in soil or on plant debris.

Click here to download a handout that with a list of fungal diseases.

Click here to view a video that explains rust: fungi that attack plants.

Bacterial Disease

Bacteria are small, simple unicellular living organisms. There are four main shapes of bacteria namely spherical, rod-shaped, helix and comma shaped. All plant disease-forming bacteria are rod-shaped, except for Streptomyces, which are filamentous. The reproduction of bacteria is simple, through cell division. Some bacteria are mobile through flagella, but mobility is limited to short distances. Bacteria multiply quickly if food sources are available and may reach a million within hours. They occur in almost all plants.

Disease forming bacteria may survive in soil, without the presence of the host plant, whilst others disappear as infected plants are removed. The main dispersion mechanism for bacteria is rain and water, but they can also be spread through the transport of infected plant material, insects or animals.

Bacterial diseases are classified according to the symptoms they produce.

Plant Viruses

Viruses are smaller than a cell and can only multiply within a cell. Many viruses are known and a plant can be infected by more than one at the same time. The major symptoms of virus infections are: reduced growth, colour mosaics, ring-spots, stem pitting, chlorosis and leaf roll.

Viruses are classed in one of five major groups according to their morphology.

Rigid rod-shaped viruses – they are rod shaped in the range of 300 nm by 15 nm with the RNA in the centre. Tobacco Mosaic Virus is an example of this type of virus.

Flexible rod-shaped viruses –they are longer and thinner than rods. An example is beetroot yellow disease virus.

Icosahedric shaped viruses – the surface consists of 20 identical flat planes, with the RNA is in the centre. (e.g. Cucumber mosaic virus)

Bacillus-shaped viruses – these are the bullet-shaped viruses, such as the Lucerne mosaic virus.

Viruses with double string RNA – some viruses such as the rice dwarf virus have a double string of RNA.

Large membrane viruses – these are almost spherical structures and are surrounded by a membrane

Important facts about viruses:

• Viruses are not transmitted through rain or wind.
• Virus transmission requires a vector.
• Viruses can be transmitted through movement of plant sap from infected to uninfected plants, such as in grafting.
• Viruses are transmitted through infected seed stocks, pollen, fungi and especially insects.

The most virulent plant diseases are transmitted through insect vectors.

Click here to download a handout with a descriptive list of typical crop diseases in SA.

Disease Management Strategies

The control of plants diseases rests on five basic principles; exclusion, eradication, protection, resistance breeding and certification.

Exclusion of a disease is based on excluding a disease from an area where the disease in not yet present. Total exclusion is not practical, but it can be achieved to some extent through using pathogen free propagation material. For this to be implemented fully, a well-managed certification scheme is required.

Eradication – eradication of a pathogen is targeted at the survival of the pathogen and is aimed at eliminating of the pathogen-forming area.

Protection – focuses on the protection of the plant against the pathogen. Protection places a barrier between the crop plant and the pathogen by the implementation of a chemical spray programme for a crop.

Resistance breeding – is the process where the genetic composition of the crop is manipulated so that the crop resists the attack from a pathogen.

Certification – includes introducing quarantine measures, as well as certifying propagation materials as disease free. (Limiting the transportation of potentially infected material from a high risk to a low risk area is also included in the certification system.) The success of a certification system relies on good management and the introduction of an inspection system.

Chemical Control of Disease

Chemicals used in the control of fungi are called fungicides. Chemical measures are introduced to aid in eradication of diseases, protecting the crop against diseases and curing the crop from an existing disease.

Eradication of plant diseases involves the control of the target pathogen while the pathogen is still on the outside of the crop plant. This is being done through seed treatments and soil fumigation before planting.

Protective measures are measures taken to place a chemical barrier to the outside of the plant or a systemic compound within the plant. The chemical is applied before the pathogen is present in the plant.

Curative measures are steps taken once the pathogen has already penetrated the crop. For this type of control, the chemical must be taken up by the plant, and must therefore have either a trans-laminar action or a systemic action.

Eradicating or curative fungicides kill the disease on or inside the plant once the disease has developed. Protective fungicides form a protective layer around the seed or crop, killing fungal spores during their germination.

Fungicides are divided into three major groupings:

Fungicides are categorised according to their chemical similarities.

Copper containing compounds – are used against a range of bacteria and funguses. It has a contact action and must therefore be used with care to avoid crop damage. These compounds are not easily washed from the leaves, thus providing a long lasting protection.

Sulfur containing compounds – are available in two forms, either as sulfur (in wettable or powder form) or as polysulfide-sulfur. The effect of sulfur on pathogens is primarily by contact and is effective against powdery mildew. It is used for controlling powdery mildew on apples and peaches, scab on apples and anthracnose on grapes.

Dithiocarbamates – are the most important group of fungicides with a contact action. Many are broad-spectrum fungicides such as Mancozeb, Maneb and Zineb. Thiram is used on stone fruit and other orchard species. Thiram is an important compound for seed treatment and can also be used as soil treatment in seed-bed preparation.

Aromatic compounds - are contact fungicides and act by binding –NH2 and – SH groups. Quintozene is a soil fungicide against soil pathogens as Rhizoctonia, but is ineffective against Pythium. Dichloran is used against rotting, including post harvest rot in peaches. Dinocap is used for powdery mildew in a number of crops.

Heterocyclic compounds – are broad spectrum contact fungicides such as Captab, Folpet, Difolatan and Iprodione.

Acylalanines – are systemic compounds with residual action. These include metalaxyl, curative against Phytophtera and oxadixyl and used for controlling downy mildew in grapes.

Benzimidazoles – systemic, broad spectrum fungicides used in foliar sprays, seed dressings and soil drenches. Examples include Benomyl and thiabendazole. (Look out for resistance development.)

Oxathiins – systemic compounds for use against smut and rust fungi and Rhizoctonia. The compounds include Carboxin and oxycarboxin.

Morpholines - Preventative and eradicating foliar applied compounds used for powdery mildew. Include dodemorph and tridemorph.

Organic phosphates – systemic fungicides, active for prolonged periods and used as sprays and fumigants. Examples are Fosetyl-al which is effective against Phytophthora and Pythium.

Pyrimidines – inhibit sterol synthesis and include bupirimate and fenarimol.

Triazoles – are systemic compounds with a protective function. Includes triadimefon, bitertanol and propaconazole.

Before a fungicide is applied, the user must be sure of the identity of the pathogen in question. Once this is identified, the user must make use of a registered fungicide ensuring it can be used on the specified crop and that it will be effective against the pathogen in question.