2. Classification of disease based on symptoms (with one example of the following): Canker, Downy mildew, Mosaic

 2. Classification of disease based on symptoms (with one example of the following):

Canker, Downy mildew, Mosaic.

Canker :

Widely distributed throughout the United States, bacterial canker is most common on cherries and plums, but may also affect apricots and many other kinds of stone fruits.

 Suspect this plant disease if sunken, water-soaked or “gummy” lesions form on the trunk or twigs.

 When trees begin active growth in the spring, a sour smelling sap may ooze from these wounded areas.

 The cankers become darker than the surrounding healthy bark, and the underlying tissue is reddish-brown to black and moist.

 If cankers girdle the branches or trunk, the leaves above the diseased area curl and turn yellow.

 Growth stops and the branch or tree will eventually die.

The bacterium that causes canker, Pseudomonas syringae, enters trees through injured bark or an existing wound, such as a pruning cut on a twig or branch. 

Frost damage in the spring may promote additional infections. 

Bacterial canker infections occur during fall, winter and early spring (during cool, wet weather) and are spread by rain or water, and pruning tools. 

The bacteria overwinter in active cankers, in infected buds and on the surface of infected and healthy trees and weeds.

Citrus canker is a citrus disease caused by the bacterium Xanthomonas axonopodis. While not harmful to humans, canker significantly affects the vitality of citrus trees, causing leaves and fruit to drop prematurely. A fruit infected with canker is safe to eat, but has reduced marketability as fresh fruit.

Treatment

1. Prune (to cut off branches from a tree, bush, or plant, especially so that it will grow better in the future) flowering trees during blooming when wounds heal fastest.
2. Remove wilted or dead limbs well below infected areas.
3. Avoid pruning in early spring and fall when bacteria are most active.
4. Treat all pruning cuts immediately  and make sure to disinfect your pruning equipment — one part bleach to 4 parts water — after each cut.
5. If using string trimmers around the base of trees avoid damaging bark with breathable  tree wrap to prevent infection.
6. Brush bark with white latex paint diluted with water to reduce bark-damaging temperature fluctuations.
7. Research has shown that  copper fungicides   have some success against this stone fruit problem. However, results are inconsistent.
8. Remove weeds and grass from around the base of young trees to improve air circulation and keep the trunk and crown dry.

 Downy mildew :

Downy Mildew, caused by Pseudoperonospora cubensis, is an oomycete that is not a true fungus. It thrives in wet or very humid conditions as a water mold. Downy mildew can infect all cucurbits including cucumber, melon, pumpkin and squash.

Downy mildew is commonly caused by members of the oomycete genus Sclerospora, but other pathogens include species of Bremia, Peronospora, Phytophthora, Plasmopara, and Pseudoperonospora.

Downy mildew (Plasmopara viticola) affects many plants and appears as yellow to white patches on the upper surfaces of older leaves. 

On the undersides, these areas are covered with white to grayish, cotton-like fungi.

 These “downy” masses are most often noticed after rain or heavy dew and disappear soon after sunny weather resumes. 

As the disease progresses leaves may eventually turn crisp and brown and fall off even though the plant has ample water.

Downy mildew occurs in cool, moist weather usually in early spring or late fall.

 Spore production is favored by temperatures cooler than 65˚F. and by relative humidities approaching 100%.

 This disease overwinters on plant debris and in the soil.

 Fungal spores can be carried by insects, wind, rain or garden tools.

Treatment

The best way to prevent downy mildew is to avoid the environmental conditions that favor the disease.

1. Prune or stake plants and remove any weeds to improve air circulation.
2. Water in the early morning hours,  to give the plants time to dry out during the day.
3. Keep the ground under infected plants clean during the fall and winter to prevent the disease from spreading.
4. Remove and destroy any plants with serious infection .
5. Choose resistant varieties whenever possible.
6. Downy mildew is comparatively easy to control on most plants when the foliage and fruit are kept protected by a copper spray . 

Mosaic:

Mosaic, plant disease caused by various strains of several hundred viruses. 

A number of economically important crops are susceptible to mosaic infections, including tobacco, cassava, beet, cucumber, and alfalfa.

Affecting a wide variety of horticultural and vegetable crops — roses, beans, tobacco, tomatoes, potatoes, cucumbers and peppers.

mosaic is a viral diseases found throughout the world.

Plant viruses can be difficult to detect as symptoms look similar to many nutrient deficiencies and vary depending on the age of the plant when infection occurs. Look for:

• Yellow, white or green stripes/ streaks/ spots on foliage
• Wrinkled, curled or small leaves
• Pronounced yellowing only of veins
• Stunted growth and reduced yields
• Infected fruit appears mottled and develops raised “warty” areas.
• 
  
• Mosaic virus overwinters on perennial weeds and is spread by insects that feed on them. 
• Aphids, leafhoppers, whiteflies and cucumber beetles are common garden pest   that can transmit this disease. 
• 
  
• Soil, seed, starter pots and containers can be infected and pass the virus to the plant. 
• 
  
• Cuttings or divisions from infected plants will also carry the virus.

• Treatment

There are no cures for viral diseases such as mosaic once a plant is infected. 

As a result, every effort should be made to prevent the disease from entering your garden.

1. Fungicides will NOT treat this viral disease.
2. Plant resistant varieties when available or purchase transplants from a reputable source.
3. Do NOT save seed from infected crops.
4. Spot treat with least-toxic, natural pest control products, such as  safer soap, bon-neem  to reduce the number of disease carrying insects.
5. Remove all perennial weeds, using least toxic herbicides , within 100 yards of your garden plot.
6. The virus can be spread through human activity, tools and equipment. Frequently wash your hands and disinfect garden tools, stakes, ties, pots, greenhouse benches, etc. (one part bleach to 4 parts water) to reduce the risk of contamination.
7. Avoid working in the garden during damp conditions (viruses are easily spread when plants are wet).
8. Avoid using tobacco around susceptible plants. Cigarettes and other tobacco products may be infected and can spread the virus.
9. Remove and destroy all infected plants.

Reference:

http://www.nou.ac.in/Online%20Resourses/30-8/botany4.pdf

https://crec.ifas.ufl.edu/extension/canker/pdf/PP13600.pdf

http://plantclinic.cornell.edu/factsheets/treecankers.pdf

https://forestry.usu.edu/files/webinars/canker-disease-slideshow.pdf

http://horticultureresearch.net/jah/2003_5_1_52_60.PDF

http://ecoursesonline.iasri.res.in/mod/page/view.php?id=11308

https://www.planetnatural.com/pest-problem-solver/plant-disease/downy-mildew/

https://www.planetnatural.com/pest-problem-solver/plant-disease/mosaic-virus/

https://www.planetnatural.com/pest-problem-solver/plant-disease/bacterial-canker/