Some important types of plant pathogens

Fungi can reproduce simply by fragments of the hyphae dropping off, but usually reproduction is by production of spores. Spores can be compared to the seeds in ordinary plants, but they are microscopic and occur in vast numbers. Asexual spores are usually important for the spread and increase of a population or epidemic. Sexual spores are important for fungal survival. 

Not all fungi have the same mechanisms for spread and survival this knowledge 

The virus was discovered in the nineteenth century. It is a very small sub-cellular organism and caimot be seen using a normal light microscope only by using an electron microscope can it be seen and identified. Serological and molecnlar techniques are now conunonly used to aid identification. Viruses consist of nncleic acids (DNA or RNA) nsually surrounded by a coat of protein. Some vimses can cause the host cell to produce various crystalline stractures which can also be nsed to aid identification, as can shape of the outer vims coat. 

Once a vims is in a host plant it causes the host cell to produce more nucleic acids and proteins from which more vims particles are formed which can then move around the plant in the phloem. All virases are obligate parasites. They are not known to exist as saprophytes. The vims is generally present in every part of the infected plant except the seed, pollen and often the apical meristem. Therefore, if part of that plant, other than the seed, is propagated, the new plant is itself infected, e.g. the potato. The tuber is attached to the stem of the irrfectedplarrt, arrd infection is carried forward when the tuber is planted as seed . 

Bacteria are very small single cell organisms, orrly visible raider a microscope. They are of a variety of shapes, but those that cause plant diseases are all rodshaped. About 12% of the identified bacteria are plant pathogens. Like fungi, bacteria feed on both live and dead material. Although they are responsible for many diseases of hrrmans and hvestock, in arable crops in the UK they are usually of minor importance compared with fungi and virases. 

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