Last time we talked about root rot, and how one of the main symptoms is a plant that wilts down, never recovers and eventually dies. We mentioned that there were also other kinds of wilts including ones vectored by insects. While these wilts are also caused by bacteria and fungi, they are not quite the same as wilts induced by root rot. However, root rot and wilts caused by soil organisms are not always easily distinguished.
Oftentimes with plants that succumb to root rot, there are subtle symptoms such as yellow leaves that indicate there is a problem before the plant wilts down and dies. If these signs are noticed and heeded soon enough, the plant can likely be saved.
Unfortunately, with wilts that aren’t due to root rot, there is typically not a lot of advance notice. Nor is there much you can do for the plant once the symptoms have appeared. Wilts may kill the plant rapidly, or in the case of some woody plants, the plant may live for years before it dies. These wilts are often vectored by insects, however they can also persist in soil around infected plants and infect new plants via that route.
Insect vectored wilts are quite commonly seen.
Example of a common insect vectored wilt of a zucchini plant. Erwinia Tracheiphila-5362952 by Howard F. Schwartz, Colorado State University: Creative Commons Attribution 3.0 Unported
One of the places you are likely to have experienced this type of wilt is in your vegetable garden with cucurbit family plants such as zucchini, cucumbers or melons. Most vegetable gardeners have had a beautiful lush zucchini plant just starting to set fruit that wilted down one day and then gradually faded away taking the promised fruit with it.
Wilts are not exclusive to cucumber plants. They affect a huge range of plants from annuals to perennials, to shrubs and trees. Some commonly affected plants include corn, beans, tomatoes, dahlias, strawberries, impatiens, mums and we could keep going on and on… Wilts also encompass some exotic diseases you might have heard about recently including Oak Wilt, Dutch Elm Disease and Vascular Streak Disease.
Insect vectored wilts are passed from plant to plant similarly to how ticks spread lime disease. An insect bites a plant that has the wilt bacteria or fungus, then it goes on to bite a plant that isn’t diseased depositing bacteria or fungal spores in the new plant. The damage done to the plant by the insect bite is inconsequential, and you may not ever even have seen the insect feeding on your plant. Unfortunately, once this occurs and your plant contracts the wilt organism, it is doomed to die.
The damage done by the introduction of bacteria or fungi is devastating for the plant.
Verticillium Wilt Infected Tree by USDA Forest Service-Northern and Intermountain Region, bugwood.org Creative Commons Attribution 3.0 United States
Plants are in large part constructed of water conducting straws called xylem and phloem. Xylem and phloem are responsible for transporting water and nutrients throughout the plant. You can think of a plant stem like a bundle of straws. The wilt organism starts growing inside one of these straws and clogs it up or actually dissolves the plant tissue so water can no longer flow. It is not a big deal for the plant if one straw gets damaged, the plant has lots of xylem and phloem so that if one gets damaged others can compensate.
Unfortunately, as we all know, bacteria and fungi grow quickly. The wilt organisms rapidly spread from their initial location where the insect bit the plant, and before you know it, they have completely clogged all the xylem and phloem tubes. Sometimes an infected plant might look fine when the soil moisture levels are very high, and then rapidly die when the weather is hot and dry because it had too much damaged xylem to survive stress.
When the plant can no longer take up water from its roots, because too much xylem is damaged by the wilt organism, it will wilt as if under drought stress, because it is regardless of the water supplied. The aerial portions of the plant will rapidly desiccate and die, almost as if you had cut the plant off and thrown it on the ground. In the case of a zucchini plant, the plant typically dies within a few days of showing symptoms. In the case of some trees, they may grow slowly and lose more and more branches until they die.
As you can imagine, this is virtually impossible to treat once the symptoms have shown up.
Striped Cucumber Beetle by Katja Schulz: Creative Commons Attribution 2.0 Generic
So what can be done about wilts? You are left to try to prevent them and mitigate damage after they arrive. Prevention of insect vectored wilts is as you might imagine simply keeping insects with their dirty mouths off your plants. This is of course much easier said than done. However, pest control is the best way to limit wilts in your garden. It is important to try to limit exposure to pests known to commonly transmit wilts, so for example, don’t let striped cucumber beetles and flea beetles, which commonly vector wilts, run rampant in your garden.
If a wilted plant does show up in your garden the best thing you can do is to remove and destroy the affected plant immediately. If you have a squash plant in your garden die from wilt, remove it. Don’t let sit there and fade away in hopes that it will recover. It is not going to. Don’t give insects the opportunity to feed on that wilted plant and then feed on the nearby healthy plants. Therefore, don’t dump a wilt infected plant in your compost heap where it could still be fed on by insects or over winter pests.
Also, use gardening tools with care. Don’t use your pruners to trim off dead portions of an affected plant and then turn around and use the pruners on healthy plants. If you do this, you will be spreading the disease to other plants. You are creating an injury and placing the pathogen into the injury exactly as a insect vector does. Sterilize your prunes between cuts and plants using something like bleach or alcohol. Bleach is corrosive, so if you use bleach, be sure to thoroughly wash and oil your pruners before putting them away.
Do not plant highly susceptible plants where other plants have died from wilt.
Phytophthora kernoviae – Rhododendron infection by Forestry Commission: Open Government Licence
As we mentioned some of these wilts can remain in the soil near where infected plants are/were. Common ones include generalists such as fusarium and verticillium wilts. Often these wilts will lurk in the soil waiting for an opportune time to attack a new host. This may occur for example in a poorly drained portion of the garden in an unusually hot wet summer.
As an example, verticillium wilt can remain in the soil for up to 25 years if it gets established. This generalist can affect hundreds of different plant species. Therefore, if your maple tree succumbed to what you suspect to be verticillium wilt, don’t replace it with another maple or you will likely get the same results. This is one of the reasons crop rotation is a recommend practice. Rotation between susceptible and non-susceptible plants giving pathogens time to die before you return to the susceptible crop.
Lastly, if you have wilts running rampant in your garden, it is likely a good indication that you do not have very healthy soil. In addition to planting resistant plants, you should work towards improving the health of your soil. As we mentioned last time in our discussion of root rot, overly wet, poorly aerated soils create the perfect environment for pathogens to flourish. These sorts of conditions also create weak plants that are more susceptible to disease problems. Sometimes something as simple as improving the drainage or texture of your soil can go a long way towards reducing problems and improving plant health.
We hope this give you some insight into the mysterious ‘sudden deaths’ of healthy thriving plants that we sometimes see.
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