Disease in plants is defined as any condition in which the use or structure of any part of the plant is not normal.

Parasitic diseases are caused by the actions of Bacteria, Fungi or Viruses. There are many types of these diseases and some of the more common ones are given below with examples from farm crops.

  • Spot Diseases: This is caused by attacks of bacteria and fungi causing characteristic spots on the leaves and stems. Grey leaf spot in maize is a well-known example.

Figure 1: Grey leaf spot on maize.

Source no-tillfarmer

  • Blight: This disease kills off large areas of leaves and stems. The common example being the blight which attacks potatoes and tomatoes and kills the plants completely.

Figures 2 and 3: Shows blight on tomatoes (left) and potatoes (right).

Source: ncalternativecropsandorganics.blogspot           Source: greensideup.ie

  • Cankers: This is mainly a disease of woody tissue i.e.: trees and the mature stems of plants. Bacterial canker of tomatoes causes shrinking and cracking of the mature stems of the plants.

Figures 4 and 5: Shows bacterial cankers on tomato plants.

Source: maine                                                                                Source: maine

  • Wilts: causes the whole plant to wilt due to infection of the roots which is then carried throughout the whole plant.

Figure 6: Bacterial wilt on potatoes.

Source: organicfarmermagazine

  • Dampening off: normally affects seeds and young seedlings. The pest attacks the seeds in the ground or the seedlings just where they are coming through causing the seedling to fall over. Soil stain in tobacco.
  • Rots: Wet rots causes a very rapid disintegration of the plant and the whole plant goes rotten. Dry rots are often hard and found in storage tissues. Cob rot in maize caused by the fungi Diplodia and Fusarium are good examples of dry rot.

Figures 7 and 8: Fusarium stalk rot in maize plants (left) and fusarium cob rot.

Source: pioneer                                                                                         Source: flickriver

  • Mildews: Powdery mildew is a white powdery layer which covers stems and leaves. Powdery mildew of wheat. Downy mildew is a white or light purple powder usually on the underside of the leaves. Downy mildew of sorghum.

Figures 9 and 10: Shows powdery mildew on pumpkin (left) and downey mildew on pumpkin (right).

Source: seedquest                                                                            Source: plantdoctor.pbworks

  • Rust: appears on leaves and stems as red/brown/orange dots which run together and spread. Leaf rust of wheat.

Figure 11: Leaf rust in wheat showing brownish/orange dots.

Source: texaswheat.

  • Smuts: black, powder masses mainly on cereal crops. Often the whole seed head is just a mass of black powder. Loose smut on wheat.

Figure 12: Loose smuts on wheat shows a black mass on the seed head

Source: ramadan-karim

  • Stunting: many virus infections cause stunting and poor growth. Rosette virus of groundnuts.
  • Mosaics: caused by viruses and plant leaves are marked in patterns of light and dark green areas. Alfalfa mosaic virus and tobacco mosaic virus.

Figures 13 and 14: Show mosaic viruses; Alfalfa mosaic virus on potatoes (left) and Tobacco mosaic

virus on tomatoes.

Source: commons.wikimedia                                                                  Source: universityofillinoisplantclinic.blogspot

  • Leaf Roll: is also caused by a virus which causes the leaves to roll upwards. Leaf roll in potatoes.

Figure 15 (left) Shows a potato plant infected with the leaf roll virus next to a normal potato plant.

Figure 16 (right) Shows a close up of the virus.

Source: quirkyscience                                                                 Source: therecycledgardener.blogspot

  • Replacement of Floral Parts: Some diseases are not easily seen because they replace the flower and cannot be seen until the plant is in full flower. An example is Ergot in rye and wheat in which the flowering part of the rye or wheat is replaced by a small highly poisonous nodule.
  • Increase in Size of Tissue: Some diseases cause increased cell division and growths appear in the form of bumps. Galls are an example and some of the large bumps that appear on trees.

Figures 17 and 18: Shows ergot fungus on wheat.

Source: northernhorse                      Source: flickriver

Figures 19 and 20: Shows club-root in cabbages which is an increase in the size of tissue cells

Source: moaf                                                                                     Source: gardener.wikia


There are a number of ways in which parasitic diseases can be carried from plant to plant. Wind and Water will carry the spores of fungi, and both bacteria and viruses from infected plants to those that rapidly become infected: this includes rainwater splash as well as running water.

Insects are very important and common method of transmitting diseases, especially the Aphids which feed on infected plants and then hop onto other plants carrying the disease with them. Leaf streak in grass and maize is carried by aphids.

Soil-Borne: diseases are those which lie in the soil from an infected crop and then attack the next crop which is planted in that same area of soil.

Some diseases can remain in the soil for 4 – 6 years and still be active and attack a crop. This is one reason why crop rotations are so important in farming.

Seed-Borne: diseases are carried from one crop to the next by infected seed. They can be carried either on the seed coat on the outside of the seed, or inside the seed itself. One way of avoiding this type of disease is to buy ‘certified’ seed which means seed which is certified to be free of disease. This is important with potatoes which can transmit Mosaic and Leaf Roll through the seed.

Man is an important factor in transmitting disease, often by moving spores and bacteria on his clothes and feet. This can spread disease from place to place on the same farm or from one farm to another. It can even be spread from one country to another and many countries do not allow people to bring plant material into their country.

Many plant diseases are affected by environmental factors with certain conditions being ideal for one disease and poor for another. Root rots appear in cool wet conditions, Potato Blight is helped by long wet spells of weather and Powdery Mildew of Peas requires conditions to be hot and dry. Alternaria of Tobacco likes a soil high in Nitrogen, and Wildfire in Tobacco is encouraged by a deficiency of Potash.

Figure 21: Shows ways in which diseases can grow and be spread.


These are diseases, or abnormal growth caused by poor management of the crop rather than any organism or infection. The major factors which cause abnormal plant growth are Soil Conditions and Cultural Practices.

  • Water: The amount of water supplied to a crop is most important. Too little causes stress and wilting, leading to a delay in growth. Too much causes waterlogging of the crop, a yellowing of the leaves and stunting. This can be overcome by draining the soil or growing the crop on ridges as in the case of vlei maize.
  • Soil Structure: Poor soil structure prevents good root penetration, reduces the water- holding capacity of the soil and the amount of air in the soil. Soil Structure can be improved by adding organic matter, either in the form of farmyard manure, by ploughing in crop residues and using grass breaks in the rotation. This is most important on sandy soils.
  • Nutrients: Deficiencies or excesses of plant nutrients cause poor growth. This can be overcome by applying the right amount of fertiliser at the right time.
    • Too much fertiliser can cause as much damage as too little. Soil should be analysed to see which nutrients are deficient and if any trace elements such as Boron are in short supply.
  • Acidity: Soils which are too acid will prevent the best growth and it is up to the farmer to make sure that if his soil requires lime it is applied and worked into the soil. All soils should be tested for pH before the crop is planted. Acid soils have a serious effect on some crops causing greatly reduced yields.

Temperature: Most crops will scorch above 38°C. The other factor which affects crops is frost.  Coffee is particularly prone to frost damage and should not be grown in areas where frosts occur in winter. Wind, hail and heavy rain are other climatic factors which will damage crops and, in some cases, destroy them altogether.


This covers all the husbandry practices which the farmer should control to the best of his ability. This entails accurate application of fertilisers, herbicides and insecticides, sowing the correct amount of seed, cultivating the crop and removing the competition of weeds. Neglect of these practices means bad farming, reduced yields and poor returns.


Many diseases can be controlled or avoided altogether if the farmer takes certain precautions and follows good farming practices. Some of the methods which can control diseases in crops are given below.

Rotations: The rotating of one crop with others helps to break the cycle of infection. Many diseases are specific to one crop as they only attack one type. If that crop is grown on the same piece of land year after year the disease that attacks can survive and increase until they reach a point where they destroy the crop completely. It is always a good idea to have a good rotation of crops so that a build- up of disease is avoided.

Ploughing-in of Crop Residues: Most disease organisms are destroyed by this method except, the soil-borne diseases. If the crop residues are burned before ploughing, all diseased material is destroyed. Ploughing should be done carefully so that all trash is buried in the soil and allowed to rot away.

Weed Control: Many diseases which are carried by insects will survive on weeds and are then carried to the crop as it starts growing. This is prevented by weeds being removed from arable lands. Aphids will live on the Apple of Peru and then carry disease to a growing crop.

Certified Seed: Buying good seed that has been certified free of disease will help prevent trouble. Although it may be more expensive it is well worth the extra money.

Rouging: This means going through growing crops, pulling out any infected material and burning it. This prevents disease from spreading throughout the crop.

Chemical Sprays: These can be seed dressings which destroy disease carried by the seed, systemic sprays which are absorbed by the plant and spread throughout the tissues giving protection, contact sprays which stick to the surface of the plant, soil sterilisation as is done on tobacco seedbeds which kill any disease in the soil and the treatment of grains in storage. It is most important to apply the right quantity of spray and at the right time in the growing period of the crop. Sprays are expensive and dangerous so that great care is needed to get the best affect.

Time of Planting: Diseases can be avoided by growing the crop when conditions are unfavorable for the spread of diseases. Wheat is grown in the winter to avoid Leaf Rust, and Bushy Top is not common in an early-planted crop.

Hygiene: The transmission of disease can be avoided by washing and cleaning equipment and hand tools, overalls, clothes, hands and boots. Smoking must be avoided when working in tobacco seed beds.

Healthy Seedlings: Always select healthy seedlings to plant out as there is no point in putting diseased plants into a land. They will not grow properly and infect other healthy plants.

Irrigation: Overhead irrigation causes splash and wets the leaves of the crop which favors the spread of disease in certain crops. Flood irrigation is best for tomatoes as wet leaves help infection by blight.

Spacing: The climate inside the area enclosed by a crop is called the microclimate and is usually hotter and wetter than the air outside. This favours the spread of disease so movement of air through the crop is important.

Fertilisers: It is important to apply the correct amount of fertiliser. Over fertilization causes dense growth affects the microclimate and helps the spread of disease.

Resistance: Plant breeders spend much of their time breeding varieties which are resistant to certain diseases. Use these varieties and keep yourself informed about new varieties and also about those which have been withdrawn because they have lost their resistance.