Tomato Diseases Part 2: Wilt & Rot
Last time we looked into some of the most harmful tomato diseases, like early and late blight, and hopefully you have been able to keep your tomato plants – and garden as a whole – disease free since. Today we’ll build off of last week and discuss a couple of wilting diseases, a disease that creates harmful spotting on the leaves and one that can cause deep, unsightly lesions, and all of these are caused by fungi. Let’s jump right in.
Found worldwide and caused by the soil borne pathogen Fusarium oxysporum, fusarium wilt infects plants by entering through their roots. This fungus reduces the roots’ ability to absorb and supply the plant with water, and this can leave the plant significantly dehydrated. Growers can purchase tomato varieties that are resistant to fusarium wilt, but no plant is completely immune to the fungus.
One of the most frustrating aspects of fusarium wilt is that many times it won’t actually kill the plant. This causes growers to put extra effort into trying to boost a plant’s growth and productivity when, in fact, it is a futile effort. In order to avoid this useless labor, the ability to identify fusarium wilt is crucial. When a plant is infected it will actually wilt. The leaves will turn yellow and start to droop, even if the plant is properly watered. The wilting usually starts from the bottom of the plant and works its way up, and eventually causes the leaves to fall off.
Other than using resistant varieties, growing in healthy soil is essential to fighting fusarium wilt. Keeping the soil at the proper pH, approximately 6.5-7.0, can limit infections. For those growing in pots, sterile potting soil is a great place to start. This will ensure that the soil doesn’t contain any harmful fungus, bacteria or other pathogens. Frequently cleaning growing tools will also help. This is especially important if tools are used at different locations, as the fungus can persist on the tools if they aren’t cleaned. Being gentle while working with plants can also make a big difference. Growers unknowingly disrupt and injure the roots of tomato plants when they dig or plant near an existing plant. When the roots are damaged or broken, the fungus has the perfect entry point and can easily infect the plant. It is also recommended that growers rotate their crops every five years or so. This will give potentially infected soil an opportunity to rid itself of the fungus. Nematode populations may also increase the likelihood of plants contracting wilt, so they should be controlled. We’ll focus on nematodes next week.
If plants do get fusarium wilt, they should be removed and destroyed. Do not put them in a compost pile. This will just introduce the fungus to a new environment. If a piece of land is infected it should not be used for about five years, if not more, and any weeds should be removed. If weeds do remain, they will feed the fungus and allow it to thrive for a longer period of time.
Verticillium wilt usually occurs in cool climates and tends to affect growers in the Northeast. It is caused by Verticillium albo-atrum, and this particular disease can damage over 200 different plant species. This fungus also enters the plant through the roots where it harms the vascular system and colonizes within the plant.
Surprisingly, verticillium wilt doesn’t necessarily cause the plant to wilt initially. Instead, yellow spots first appear on the lower leaves. As the fungus progresses, brown veins start to show, and finally brown spots form on the leaves. As the fungus takes over, the plant will cease to develop and later in the season leaves may begin to wilt and fall off.
There are a few steps that growers can take to reduce the risk of verticillium wilt, and by far the best way is to maintain the health of the land. Verticillium albo-atrum loves soil that is damp and wet, so tomoatos really should be planted in soil that is able to properly drain and has low moisture levels. Other than this, rotating tomato crops every five years can help.
Septoria Leaf Spot
Septoria leaf spot is one of the most common plant diseases and it proliferates when the weather remains warm and wet over long periods of time. It is caused by Septoria lycopersici, which is easily spread via splashing water. Septoria leaf spot sets in with small, gray lesions that have dark borders form on the leaves. The leaves then begin to fall off and as the fungus ascends the plant total defoliation may occur.
The best way to reduce the risk of leaf spot is to buy disease-free seeds, and to make sure that there isn’t standing water around plants. Spacing the plants properly will promote circulation and reduce the amount of water and moisture that is found on the plants. Rotating crops every three years – which is less than other fungal diseases – will also help to stop septoria leaf spot. If a plant does become infected, removing and discarding the leaves should be done as soon as possible. The leaves carry spores, and if they come loose from the plant the spores will be able to spread over a much greater distance. After gathering the leaves, it’s important to completely remove and destroy the plant.
Sour rot is another soil borne fungus, caused by Geotrichum candidum, and this disease causes the actual fruit of the plant to decay. Geotrichum candidum is a form of yeast that is easily spread, because it can live not only in the soil, but also on plant debris and the tools and handling equipment used for growing. It is also sometimes passed through water or the wind.
Tomatoes should always be handled with care. After all their visual appeal does correlate to the price they will fetch at market, but cracks and injuries can be an entrance point for the sour-rot-causing fungus. When a tomato has been infected by sour rot, cuts and lesions form on the fruit. Initially, these blemishes are watery, but over time they become filled with a white, fluffy fungus. Although it may look soft to the touch, these lesions remain firm unless another infection takes place.
When trying to prevent sour rot a clean environment is at the foremost importance. Regularly draining and cleaning dump tanks, storage containers and tools – especially if they’ve been used at another location – will reduce the spread. Keeping plants warm and dry can also help. Plants should be properly spaced so that air can flow and they dry quickly. If plants do become infected with sour rot they should be removed immediately. Infected fruit cannot be put in a holding container with other healthy fruit, because it will spread throughout the healthy tomatoes.
These past two articles certainly don’t recap every disease that can affect tomatoes, but these are some of the more common diseases. Next time we’ll take a look into pests that can destroy healthy crops. Have a good week everybody.