It is important to determine if a plant is suffering from the results of disease, insect infestation or nutritional deficiencies. Sometimes a diseased plant will also fall prey to insects, making it difficult to tell which occurred first. There are many plant diseases, and these can vary by region. Check with your local horticultural extension service, as they are usually up-to-date on recent disease outbreaks in your area.
A plant that is much smaller than it should be could have a disease. Look for signs above ground first. Eliminate the possibility of a soil pH problem, as pH can affect the plant's ability to take in nutrients, turning the plant yellow in the process. Nutritional deficiencies pave the way for Botrytis cinerea and other fungal diseases. This fungal disease can show up on a wide variety of plant types and can infect any area of the plant. It causes brown areas, and when touched, it feels as if dust were coming off of the plant.
Fungal diseases like anthracnose appear as dead or brown patches in the leaves. Another fungal disease, powdery mildew, manifests itself as fuzzy white patches on the foliage. A fungus called rust appears as round orange spots. A common virus, tobacco mosaic, causes a distinct mosaic design throughout the leaf. A virus will be absent of the spores or dust that comes off of the plant when touched. Viral pathogens can only be seen under a microscope. The result of both viral and fungal infections will be dropped leaves.
Collapsed Stems and Twigs
When the cell walls are damaged from disease, it shows in the plant stem. The usual pathway for water and nutrients through the stem becomes shriveled and blocked. This can be a sign of viral or fungal infection. The twigs can be plagued by the same infection as the leaves, so the visible signs will be very similar. There will be sections of dark discoloration or dark spots moving up the stem.
No Flowers or Fruit
When a previously healthy plant is struggling to fight a disease, the early signs may be lack of flower or fruit. The plant's energy is going toward recovery and there is little left for new tissue. Fungal and viral diseases can affect flowers and cause them to drop prematurely. Make sure this is not due to a late cold snap. If there is fruit but it is disfigured, it may be a fungus. Look for white or orange patches on the fruit. It is easy to tell the difference between insect and viral or fungal damage. Upon examination you will find insects, larvae or bite marks. Fruit with minor surface spots will likely recover on its own. If the affected areas go deep onto the flesh, it is a more serious disease.
When a plant fails to thrive for no apparent reason, it could be root disease. Unfortunately, once the signs of fungal diseases of the roots such as phytophthora are visible, the fungi have already infected the stem. Dark collapsed areas begin to show at the base of the stem, and the cell walls begin to collapse. Another fungus that starts at the roots and works its way up is verticillium wilt. If the roots are mushy with no spots or patches elsewhere, it may be a case of root rot from too much ground water.