Plant diseases can be difficult to diagnose. So often, they display the same symptoms as plants that are perfectly healthy, except for stresses imposed upon them by our poor cultural practices. When a plant is diseased, it is because of a bacteria, fungus, or virus. A better understanding of each will help you diagnose and treat the problem if possible.
Not all bacteria are bad for plants and soil. In fact, most are beneficial, and there are millions! However, there are approximately 200 types of bacteria that cause diseases in plants. They are most active in warm and humid environments, so this is when you’ll see the most evidence of their presence.
There are several symptoms of bacterial infection. One is leaf spot. In this case, the bacteria that attacks the plants, produces a toxic chemical that kills the surrounding plant cells. The plant then reacts defensively by killing off the surrounding plant cells, thereby isolating the infected cells. In some cases, these dead cell areas drop out, creating what looks like “shot holes” in the leaves.
Bacteria can clog the plants ability to deliver water and nutrients to the rest of the plant. Eventually the plant begins to wilt or droop. This process can occur rapidly, and within one day, you can see a dramatic decline in your plants.
Other symptoms cause the plant tissue decline, such as in cankers and soft rot, which are sunken areas produced by dead plant tissue. In other cases, abnormal growth is the symptom, referred to as galls. Plants respond to these bacterial invasions by producing a rapid abundance of new cells. This is often evident is unusually large, misshapen growths somewhere on the plant or root.
Bacteria can spread in several ways, including insects, splashing water, other diseased plants, or tools. They enter plants through tiny openings either through damage, or cuts, but also through natural opens in the plant itself.
Once plants are affected, they can be difficult to control. Remove infected plants or parts immediately from the garden. Do not add these parts to your compost pile. Instead, destroy them. Once present, controls options are limited. Copper based sprays provide some help, but are not a cure. Bacteria are best controlled as a preventative measure, treating plants before damage is even present. Additionally, good cultural practices are always helpful. This includes sanitation of equipment, and removal of all plant debris.
Like bacteria, many more fungi are actually good for the garden. But, unlike bacteria, there are thousands of fungi that are harmful to plants. For this reason, you are likely to encounter fungal problems most often. Because fungi are present in the soil and above ground symptoms of fungal attack can appear above and below ground. These include rotting or dead roots, or large swelling on roots below ground. At the soil level, new seedling stems can rot and flop over. Above the soil line, plants can display leaf spots, mildews (white or gray powdery patches on foliage), rusts, and wilts.
Fungal spores are very small and light, and can travel great distances through the air to infect other plants or trees. They are also spread by water, animals and insects, and people.
The best way to prevent fungi from attacking your plants is to buy disease resistant varieties whenever possible. Other ways include minimizing the amount of water contacting foliage. Water at the soil level and early in the day. This allows foliage to dry out quickly, should it become wet. It’s also helpful to provide good air circulation through proper spacing between plants, and pruning.
To control fungal outbreaks, as with bacteria, remove all infected plant parts, or plants. You may also choose to apply a fungicide. There are many products available for treatment, organically (copper, sulfur, and baking soda are common) or synthetically. These treatments are best at preventing the germination of new fungal spores, so applications before outbreaks occur will provide the most effective control.
Even viruses on occasion can be beneficial, but for the most part, they are bad news in the garden. They can persist for many years, before they appear as a problem, and when they do, they often show up in one of a few primary ways. First, plant foliage may appear yellow, or they may appear as mosaic patches of yellow, light green, or white. Next, the plant may appear stunted. In addition, the plants are often misshapen or malformed. Specifically, the leaves may be rolled, or swollen or puckered, or they may be abnormally narrow.
Unlike bacteria and fungi, viruses are not spread by water or wind. Instead, they must physically enter the plant. One of the most common vectors of viruses are insects. Insects feed on infected plants and transmit the viruses to healthy plants when they feed again. Other ways include plant propagation, contact by humans, and infected seed.
Unfortunately, once infected, there are no chemical treatments for eliminating a virus. Once detected, you should remove all suspected plants. Although this can seem like drastic measures, it is the most effective way to reduce continued spread. It is difficult to prevent viruses from affecting your plants. Your best efforts will be to look for virus-resistant cultivars, provide physical barriers, such as floating row covers, or to actively eliminate vectoring pests from entering your garden.