Plant diseases can be broken down into two rather broad categories. Non-parasitic diseases are caused by effects like too much humidity, too little fertilizer or incorrect pH balance in the soil. Parasitic diseases are contagious and caused by bacterial or fungi being passed from one plant to another through tools, hands, insects or other methods of conveyance.
Powdery mildew can affect nearly all plants, but is especially found on begonias, lilacs, roses, strawberries and lawn grasses. The symptoms of powdery mildew are a white coating found on the leaves and shoots. This substance can sometimes be found on the flowers. Growing season is the most likely time of occurrence.
Damping off affects the seedlings of almost every plant. Damping off occurs when soil-borne fungi attack the seedlings. This disease can also attack plant cuttings. The symptoms are wilting, followed by the collapse of the stem. Causes of damping off include overwatering, planting when the temperature or humidity are too high and soil with a too high nitrogen content.
Leaf spot can be caused by either bacteria or fungi infection. A fungal infection results in dark, dry spots that can often be ring-shaped with smaller rings inside. The bacterial infection produces smooth, irregular spots that are sometimes surrounded by a yellow halo-effect. Causes of leaf spot include some kind of physic trauma that results in a wound through which organisms can enter. Other causes include overwatering, low light levels, poor ventilation and a sudden chill.
Rhizome rot affects rhizomes from irises to ginger. The symptoms include a soft, yellow rot at the growing point that produces a foul odor. Ultimately, this disease causes the leaves to collapse. Rhizome rot can occur at any time, but be especially aware during rainy periods.
Stem rot affects a variety of plants, but each deals with its own unique form of the disease. The stems begin to rot for no apparent reason. Stem rot is notable for the lack of any obvious signs of fungus growth on the affected area in some cases. Other plants will exhibit signs in the form of stems that become darker and turn mushy. Causes include growth in extremely warm or humid areas, overwatering and poor ventilation.
Shot-hole fungus attacks fruit plants like cherries, peaches and plum trees. Symptoms include small purple spots that appear on twigs, leaves and developing fruit. These spots begin appearing in spring and eventually turn brown. The spots sometimes drop free of the leaves and leave behind what looks like a shot-hole. The result is a reduced yield of fruit, but one that can still be eaten if peeled.
Crown gall is characterized by large knotty areas that can grow to several inches in diameter that appear at the base of the plant as well as on stems and roots. The result of crown gall can be the death of a single branch or an entire plant or it can have no effect at all. Crown gall is caused by a bacterium that inhabits the soil around the plant. It is most often caused by the introduction of a plant to the garden that is already contaminated.