Powdery Mildew is one of the most common garden diseases. The fungus usually appears as white, powdery spots on leaves or yellow irregular blotches on tomato plants. It spreads easily through the air. Leaves covered in mildew die back resulting in lower vegetable yields.

Prevention is the key to controlling this disease. Plant disease resistant varieties. Make sure your plants have adequate sunlight and airflow. Don’t water leaves late in the day.

Fungicides can be effective treatment for mildew but most need to be applied before infection occurs. The safest ones use natural types of bacteria such as bacillus amyloliquefacius or bacillus subtillus. Wettable sulfur is effective at preventing mildew but is incompatible with horticultural oils like neem oil. Once a plant is infected, horticultural oils are the most effective remedy. Do not water leaves once a fungicide is applied as this will reduce its effectiveness.

Another treatment option is to wash off infected leaves early in the morning. This will slow down the spread of the fungus.

Source:
http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html

Blossom end rot affects tomatoes, peppers, squash and other cucurbits. The blossom end turns brown and the fruit rots. The cause is a lack of calcium aggravated by an imbalance of soil moister and high salt levels. Most garden soils have high levels of calcium from adding gypsum and the problem is likely caused by the inability of the plant to update nutrients due to root issues such as nematodes or root diseases.

Be sure your soil does not dry out nor become oversaturated with water. It is not a caused by a pathogen nor can it be spread to other plants. If the plant is not healthy, remove it and plant something different in the location.

Source:
http://ipm.ucanr.edu/PMG/GARDEN/VEGES/ENVIRON/blossomendrot.html

Late blight affects tomatoes, peppers, eggplant and some other vegetables. It appears as irregular, small pale to dark green water-soaked spots that are surrounded by a zone of yellowish tissue. Lesions may expand rapidly and become brown to purplish black. Fruit discoloration usually begins on the upper side and remain firm. If you suspect that a plant has this disease, have a knowledgeable garden member confirm it.

This is a serious disease so prompt action is necessary. Pull out the plant and dispose in a plastic bag to avoid spreading the disease to other nearby plants.

Source:  
http://ipm.ucanr.edu/PMG/GARDEN/VEGES/DISEASES/lateblight.html

Symptoms on plants with cucumber mosaic diseases can vary, but in general, plants show an overall lighter color along with mosaic patterns (alternating light and dark green areas) on at least some leaves, especially on the younger leaves. Often, the main leaf vein is distorted and somewhat zigzag in appearance. Plants generally show stunting, leaf curling, and mosaic, and mature leaves can develop necrotic areas shaped like oak leaves. Fruit may be malformed and have conspicuous concentric rings or spots.

CMV affects many types of vegetables including melons, tomatoes, celery, peppers, spinach and beans.

CMV is usually spread by infected aphids or contaminated tools. In order to control aphids, you need to encourage aphid predators into your garden and to control ants.

Whenever possible, try planting CMV resistant varieties. Clear your garden of weeds with can harbor the disease. Remove infected plants as soon as possible to slow the spread of the disease to healthy plants. There are no known effective treatments for CMV.

Source: 
http://ipm.ucanr.edu/PMG/r116102011.html

Fusarium wilt is a fungus that usually enters through the roots of plants. The lower leaves on one side of a plant turn yellow or brown then wilt. Branches die back until the entire plant succumbs. The vascular tissue turns brown. Hosts include asparagus, bean, cabbage, cantaloupe, pea, tomato, and watermelon.

Gardeners often make the mistake of assuming the problem is due to lack of water or fertilizers. Overwatering or excessive fertilizer will exacerbate the problem.

There is no known cure for this disease. Try purchasing plants that are Fusarium resistant (usually has the letter “F” on the label). Plant on mounds or in raised beds in well drained soil. Rotate crops so that the same plant is not in the same location for at least 2 years. Avoid mixing undecayed organic material into the soil. Do not apply excessive nitrogen. Use calcium nitrate instead of ammonium based fertilizers.

Source:
http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/fusariumwlt.html

https://extension.umn.edu/disease-management/fusarium-wilt#cultural-control-3037561

The symptoms of verticillium are very similar to Fusarium. Leaves turn yellow, brown and wilt. The entire plant will eventually succumb. There is no treatment or cure.

Avoid overwatering and adding too much fertilizer. Plant in raised areas in well-drained soil. Purchase plants that are verticillium resistant (usually has the letter “V” on the label). Rotate crop locations.

Source:
http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/vertwilt.html

Before using fungicides, carefully read the label and follow directions.  Be sure to wear protective clothing.  Do not apply if other people are in the same area.  Check the National Pesticide Information Center for more information:  http://npic.orst.edu/

Biological fungicides are generally safe to use and have little or no effect on beneficial insects.  Bacillus amyloliquefacius and bacillus subtillus are the most common ones and are sold under various trade names.  They need to be applied every 3 to 5 days as the bacteria will eventually degrade after being exposed to air and sunlight.  Used as a preventative against many common fungi such as powdery mildew but not effective at stopping the spread once plant is infected.  Some can also be used as a soil drench to protect roots from soil borne pathogens.

Wettable sulfur is generally effective at protecting plants from powdery mildew and other air borne pathogens.  It is incompatible with horticultural oils such as neem oil.  It is not effective at stopping the spread of the disease once the plant is infected.  Sulfur is harmful to some beneficial insects such as parasitic mites.

Neem oil is an effective fungicide and can be used to slow or stop the spread of some fungi such as powdery mildew.  Incompatible with sulfur.  Neem oil can harm a few beneficial insects.  Try not to use in areas where bees are active.