Bacteria, viruses, fungi, pathogens and parasites cause crop diseases just as they cause human diseases. The organisms that cause the diseases are different, but the effect is the same in that they destroy the good cells. It doesn’t matter whether the organism attacks the roots, stems or leaves of the crop because it will weaken the crop and cause it either not to grow properly or die. When a crop has a disease in the leaves, it cannot get the food supply it needs to grow. Some organisms block the flow of water from the stems to the roots and then the roots will die for lack of water and nutrients.
Fruit trees are susceptible to fungal canker. This fungus infects the branches of the trees causing areas of the truck to become sunken and distorted. This will eventually girdle the branch causing it to die. Shoots of the fruit that are weakened by the fungus will break off in the wind. Fruit that does grow on these infected trees will develop pits.
Plum, cherry and apricot crops are susceptible to bacterial canker. An area of the trunk becomes swollen with light brown gum. This is usually near an angle in the branch. It will continue to grow until it covers the branch until it dies.
Potato crops sometimes develop blackleg when they are stored and waiting to be sold. It also affects carrots and turnips as well as potatoes that are stored in a cupboard. This bacteria cause soft rot in the cells of the crop. This disease can also affect crops growing in the field. When it does so, the leaves turn yellow and the stems will turn black and then fall off. This is a dangerous disease for potato farmers because the bacteria travel through the underground roots from one plant to another infecting each one as it does so.
Broad beans can be infected with fungal disease called Chocolate Spot, so-called because it looks like spots of chocolate on the leaves. Those crops grown in the winter are more susceptible than spring and summer crops. This disease will reduce the size of the crop, but the beans from the infected plants are still edible.
A disease called Club Root can affect vegetables growing in a garden. The roots become stubby and swollen and stunt the growth of the crops. Spores of the fungus cause a jelly-like mass to form on the leaves, which in turn reduces the amount of water and nutrients that the plants can receive.
In some cases the only way of controlling these diseases is to burn the infected crops. Rotating the crops you plant in fields each year sometimes helps, depending on the specific disease.