Plant pathology 

encompasses the study of what causes a plant disease; how a pathogen attacks the plant at the molecular, cellular, tissue, and whole plant levels of organization; how the host responds to attack; how pathogens are disseminated; how the environment influences the disease process; and how to manage plant pathogens and thereby reduce the effects of the disease on plant populations.

Fungi 

Fungi interact with plants as pathogens or benefactors and may influence yields. Fungal plant pathogens can cause enormous losses in yield and quality of field crops, fruits, and other edible plant material, and this becomes increasingly a more important issue to human health and the global economy in this century, with increasing human populations and climate change threats to arable land.

Fungi can destroy crops, and the economic consequences of this have been enormous throughout human history. Fungi produce toxins poisonous to humans and animals.

 

For example, the Great Bengal Famine of 1943 caused by Cochliobolus miyabeanus in rice, the Irish potato famine, caused by Phytophthora infestans, in Ireland in the 1840s, the southern corn leaf blight epidemic of 1970-1971 in the USA.Fungi are eukaryotic, heterotrophic, absorptive organisms with cell walls. Many plant pathogenic fungi are classified as necrotrophs. These fungi become parasitic, kill, and then feed on dead plant tissues. The following are the most harmful of pathogenic fungal species in several crops:Leaf rust, Fusarium head blight/scab, tritici blotch, stripe rust, spot blotch, tan spot, and powdery mildew caused losses higher than 1 percent globally in wheat.Sheath blight, stem borers, blast, brown spot, bacterial blight, leaf folder, and brown plant hopper did the most damage in rice.Fusarium and Gibberella stalk rots, fall armyworm, northern leaf blight, Fusarium and Gibberella ear rots, anthracnose stalk rot and southern rust caused the most loss globally in maize,.Late blight, brown rot, early blight, and cyst nematode did the most harm.In soybeans, cyst nematode, white mold, soybean rust, Cercospora leaf blight, brown spot, charcoal rot, and root knot nematodes caused global losses higher than 1 percent in potatoes.

Fungicide

The classification of fungicide development are in categorized in three major group. The most common is SDHI (succinate dehydrogenase inhibitors) or those considered to be SDHI due to their chemical structure characteristics.

SDHIs are considered as one of the largest groups of agricultural fungicides, as well as DMI (demethylation inhibitors) and inhibitors of the mitochondrial electron transport chain complex III, i.e., QoI (quinone outside inhibitors) and QiI (qui- none inside inhibitors).

Post- SDHIs fungicides are now desired as a result of successive reports of resistance of fungal pathogens to existing SDHIs. 

DMIs, sterol biosynthesis inhibitors characterized by triazole fungicide, perform a significant role in the plant diseases control because of their wide spectrum, high therapeutic impact and relatively slow development of resistance.

The second global trend is the development of fungicides with a novel mode of action and a special chemical arrangement. Other approach are based plant defense activators as a fungicide with the nutritional effect.

Nematodes

Nematodes are the most multicellular animal which over 4000 species of know as a parasitic. Surprisingly, there are about 50 million nematodes in a square meter of moderately fertile soil to 30 cm depth among which l0% are plant parasites. Plant-parasitic nematodes found in the soil and in plant roots. Most species are microscopic.Plant-parasitic nematodes attack most economically important plants in agriculture, horticulture, ornamentals. The symptoms attributed to plant-parasitic nematodes include stunting, yellowing, and wilting.The damage to plant is the not only the consequent of direct nematode attack to the root cell, but also interference with the root system, reducing their efficiency in terms of access and uptake of nutrients and water; to the unaware, nematode-affected plants present typical drought and nutrient stress symptoms.Root-knot nematodes (Meloidogyne species) are a wide-spread and diverse group of plant-parasitic nematodes. Cyst nematodes (Heterodera, Globodera, and Punctodera species) are sedentary endoparasites.Nematodes have very specific character. Some have wide host range like Aphelenchoide while some have generally narrow host range and specialized like Heterodera.Some are ecto- parasite and some are endo- parasite and some are both in different part of the plant in their feeding habit like Aphelenchoide. Most of them have large number of species.

Nematodes controlling approach

They can be managed by using different methods like; Sanitation of fields and equipment like Pots potting soil, bio-control agents Paecilomyces lilacinus, Periodic rotation, fumigation and using of clean planting materials.

The controlling methods for nematode are generally divided to fumigant and non-fumigant. Most nematicide show control effects by entering the nematode body through the surface phospholipid membrane.

Methyl bromide, a typical fumigant, had widespread effects and was widely used around the world due to its low cost. However, the use of methyl bromide was discontinued in developed countries by 2005 due to fears of ozone depletion and groundwater pollution at the Montreal Protocol meeting in 1995.

Different groups of nematidies including SDHI fungicide by acting on complex II developed and marketed by different companies in the worlds.

Bacteria

All prokaryotes have cell membranes, cytoplasmic 70S ribosomes, and a non-membrane limited nuclear region.Most plant pathogenic bacteria are facultative saprophytes.Erwinia amylovora, the bacterial cause of fire blight, will reproduce and cause increasing numbers of infections as long as moist conditions prevail.Enzymes are catalytic proteins.Cell degrading enzymes (CDEs) produced by plant pathogenic bacteria reduce plant components.Chemical control of diseases caused by phytopathogenic prokaryotes is made difficult because of short generation times.

Viruses

Plant viruses generate economic loss for farmers, producers, and consumers by adversely affecting plant growth and reproduction; causing death of host tissues and plants, sterility, reduction of yield or quality, crop failure, increased susceptibility to other stresses, loss of aesthetic value, quarantine and eradication of infected plants; and increasing the cost of control and detection programs.


Viruses are obligate parasites, ultramicroscopic and have protein coats or capsids. Virus species may also be subdivided into strains and isolates. Strains are named when a virus isolate proves to differ from the type isolate of the species in a definable character, but does not differ enough to be a new species.
Virus vectors include insects, mites, nematodes, fungi, seed, and dodder and also humans, animals.