The Mechanisms by Which Pathogens Cause Disease

Plant diseases cause a huge loose of fruits, vegetables and crops since long time ago, historically diseases causes huge loose that leads to the dead of millions of peoples on earth. During early peoples, not know the causes of diseases, they were considering the cause of diseases some mysterious agents, much myth related to the causes of diseases.	As science advance the mysterious agents of diseases discovered, peoples come to know the real causes and agents. Scientist observed fungi, bacteria and microorganisms were the causes of different diseases, but another big question arise after the discovery of casual agents, “How fungi and microorganisms cause diseases?”	Previously it became noticeable that fungi and other microorganisms were the causes rather than the results of plant disease, efforts began to understand the mechanisms by which microorganisms cause disease.
	In 1886, deBary, working with the Sclerotinia rot disease of carrots other vegetables, noticed that host cells were killed in advance of the invading fungal hyphae and a juice from rotted tissue released that could able break down healthy host tissue, when juice from rotted tissue boiled and treat healthy tissues, had no effect on healthy tissue. DeBary concluded that the pathogen produces enzymes and toxins that degrade and kill plant cells from which the fungus can then obtain its nutrients.
After deBary, many attempted to show that most plant diseases, particularly vascular wilts and leaf spots, were caused by toxins secreted by the pathogens, but those claims could not be confirmed. A 1925 suggestion that the bacterium Pseudomonas tabaci, the cause of the wildfire disease of tobacco, produces a toxin that is responsible for the bacteria-free chlorotic zone (“halo”) surrounding the bacteria-containing necrotic leaf spots was confirmed in 1934.		The wildfire toxin was the first toxin to be isolated in pure form in the early 1950s. In 1947, a species of the fungus Helminthosporium (Bipolaris), which attacked and caused blight only on oats of the variety Victoria and its derivatives, was shown to produce a toxin named victorin. This toxin could induce the symptoms of the disease only on the varieties susceptible to the fungus.
	Many other bacterial and fungal toxins were subsequently detected and identified. The toxins exhibited several distinctive mechanisms of action, each affecting specific sites on mitochondria, chloroplasts, plasma membranes, specific enzymes, or specific cells such as guard cells. In addition, several detailed biochemical studies were carried out to elucidate the mechanisms by which toxins affect or kill plant cells or by which cells of resistant plants avoid or inactivate them.

Common Rose Diseases

By Marie Iannotti, About.com Guide

Despite the best efforts to keep roses disease free, there will inevitably be period problems to deal with. Keep tabs on your rose bushes so that you can remedy the situation before it gets out of hand. Be sure to accurately identify the problem before reaching for a spray and start with the least toxic solution.

1. Black Spot: Black spot is a fungus that is very common during humid weather because it is a water-bourne disease. As its name implies, small black spots form on leaves and stems, eventually causing the leaves to drop and weakening the plant.
   Treatment: Choose black spot resistant varieties and be meticulous about sanitation. Water the roots of the rose, avoiding the foliage. Water in the morning, so that splashed leaves have time to dry off. If Black Spot is an annual problem, try a dormant spray of lime sulfur at the end of the season and again in early summer. Once Black Spot appears, it is hard to stop. Neem oil and Sprays containing Potassium bicarbonate are somewhat effective.
2. Downy Mildew: Downy mildew is a very serious disease that spreads rapidly and can defoliate a rose plant in days. It is not as common as Black Spot and favors cool, wet weather. Purple spots with yellow edges form , often on the veins on the top side of the leaves and along the stems. Pale gray fuzz can form on the under side of the leaves. The leaves will eventually become brittle and fall.
   Treatment: The good news is that Downy Mildew often clears up with the weather. To reduce the chance of Downy Mildew, practice good garden sanitation and keep the rose plants well pruned for air circulation. As with other diseases, a dormant spray may help.
3. Rust: A Rust infection is easy to spot. Small orange pustules spots form on the undersides of the leaves. This fungus can also cause defoliation. Rust is most prevalent when nights are cool.
   Treatment: Treatment of Rust is similar to treatment of Black Spot, above: Good sanitation and a preventative dormant spray after pruning. Once infected, remove all infected leaves and try Neem oil for control.
4. Mosaic Virus: Once a rose is infected with Rose Mosaic Virus, there’s not much to be done except check with the nursery for a replacement. Rose Mosaic Virus shows up as yellow mottling on leaves and deformed new growth. It can stunt growth or it can be a mild infection. If there are only a few affected leaves, the plant may continue growing and blooming fine. The really good news is that it won’t spread to your other roses.

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Plant Diseases May Cause Financial Losses

In addition to direct losses in yield and quality, financial losses from plant diseases can arise in many ways.Farmers may have to plant varieties or species of plants that are resistant to disease but are less productive, more costly, or commercially less profitable than other varieties. They may have to spray or otherwise control a disease, thus incurring expenses for chemicals, machinery, storage space, and labor. Shippers may have to provide refrigerated warehouses and transportation vehicles, thereby increasing expenses.

Plant diseases may limit the time during which products can be kept fresh and healthy, thus forcing growers to sell during a short period of time when products are abundant and prices are low. Healthy and diseased plant products may need to be separated from one another to avoid spreading of the disease, thus increasing handling costs.
The cost of controlling plant diseases, as well as lost productivity, is a loss attributable to diseases. Some plant diseases can be controlled almost entirely by one or another method, thus resulting in financial losses only to the amount of the cost of the control. Sometimes, however, this cost may be almost as high as, or even higher than, the return expected from the crop, as in the case of certain diseases of small grains. For other diseases, no effective control measures are yet known, and only a combination of cultural practices and the use of somewhat resistant varieties makes it possible to raise a crop. For most plant diseases, however, as long as we still have chemical pesticides, practical controls are available, although some losses may be incurred, despite the control measures taken. In these cases, the benefits from the control applied are generally much greater than the combined direct losses from the disease and the indirect losses due to expenses for control.
Despite the variety of types and sizes of financial losses that may be caused by plant diseases, wellinformed farmers who use the best combinations of available resistant varieties and proper cultural, biological, and chemical control practices not only manage to produce a good crop in years of severe disease outbreaks, but may also obtain much greater economic benefits from increased prices after other farmers suffer severe crop losses.

Source: Plant Pathology by GEORGE N. AGRIOS