A lot can go wrong with your tomato plants
Q: My tomato plants' leaves have varying patches of brown and I notice small holes in some leaves and others have dark spots surrounded by tan dried areas mainly on the bottom half of the plant. Why is this occurring?
A: There probably is more than one issue causing the problems you describe. As many gardeners know, it is difficult to grow tomatoes in the Kingman area and this year is especially trying because it has been unusually hot in May and June. Let's take these symptoms one at a time and look at simple solutions.
Patches of brown on the leaves could be as simple as under- or over-watering and as complex as fusarium or verticillium wilts. Planting too close together can restrict airflow and sunlight to the lower parts of the plant, causing browning of leaves. Also, yellowing and then browning of leaves at the base of tomato plants can be a natural occurrence as they age.
Proper watering requires keeping the soil evenly moist. If the top couple of inches of soil feels dry, irrigate. If it is moist, do not irrigate. Mulch with a few inches of straw, mowed grass, wood chips, plastic or a purchased product to prevent rapid loss of moisture from the soil. Recycled rubber and plastic mulches are inorganic and will not provide nutrients to the soil but will keep the roots cooler and retain moisture.
Many tomato varieties have disease resistance to fusarium and verticillium wilts. These wilts are a fungus entering the plant through soil moisture. The treatment is drastic, requiring removal of affected areas and possibly the entire plant. Then you should solarize the soil by covering with clear plastic for at least six weeks during the hottest period of the year. The good news is that many varieties of tomatoes are resistant to these fungi and treating for these problems should be a last resort.
Dark spots surrounded by tan dried areas may be a magnesium deficiency. Magnesium is needed by the plant to make chlorophyll (which is the green pigment). Chlorophyll is important because it absorbs energy from the sun and helps the plant to synthesize carbohydrates (called photosynthesis). Lightweight and sandy soils are more susceptible to this deficiency because it gets washed away. A fertilizer high in potassium can inhibit the intake of magnesium (the third number on a bag of fertilizer).
To help with a magnesium deficiency, put one tablespoon of epsom salts in the bottom of the hole at planting time and mix with the soil on the bottom. Place the plant on top. Improve sandy soils with organic soil and fertilize with low or no potassium.
Fungal, viral and bacterial diseases like Bacterial Speck or Spot, Leaf Spots (there are several types), Tomato Spotted Wilt Virus and others may also have symptoms with dark spots. These can be difficult to diagnose. An easy way to reduce the likelihood of a fungal or bacterial disease is to rotate where you put plants at least every few years. This also applies to containers, but replacing the soil each year and cleaning the containers with a 10 percent to 20 percent bleach concentration in water will also help.
Small holes in leaves can be caused by thrips, which are tiny white insects the size of a pinhead that feed and lay eggs on the leaves. You may need a magnifying glass to see them. They can also spread the Tomato Spotted Wilt Virus. If you suspect you have them, spray with insecticidal soap as an organic control (but you must spray often) or another type of insect control. Be sure to follow directions on the package.
As discussed above, there are many tomato diseases. Implementing the simpler recommended solutions can prevent more complex fungal, bacterial and viral problems that require more drastic solutions.
Q: It's been excessively hot for much longer than usual. Is there anything I need to do to for my plants to help them?
A: That's a good question. Plants can't get up and come into the air conditioning when the temperatures soar. But just as there are precautions against and indicators of heat stress, so are there things we can do for our gardens.
The first very important one may be counter-intuitive: Do not overwater. When it's really hot, our tendency is to water more. But just as we can't drink a gallon at one time, so too can plants' roots be too wet, literally drowning them.
Yes, it's even more difficult not to water when the leaves of the garden look wilted in the middle of the day - but if you think about it, we too look wilted when we're out in the heat in the middle of the day. That can be a natural response. Watering in the middle of the day is really bad for plants for several reasons. If the water stays on the leaves, it can act as a magnifying glass and literally burn them. You likely will lose more water to evaporation and increase your water bill. Also, since the mid-day slump may just be that, and not an indicator of parched roots, you are more likely to increase the soggy root problem, which can be fatal. So wait until the main heat of the day has passed and see how much the plant has revived before watering.
Watering in either early morning or just before sunset is best. Even then it is wise to use the "finger test" for potted plants or a probe that can measure moisture for deep-rooted trees or garden plants. Put your finger or a probe into the soil and see how damp it is. If it is still damp at the end of it, then unless you have a plant that needs to stay constantly moist, you don't need to water.
Another temptation to resist is fertilizing during the heat. People frequently say that since heat is so stressful, plants need extra fertilizer for energy and growth. However, just as humans need and eat cooler, lighter meals during heat, so too do plants. Even in cool times, too much nitrogen fertilizer has a possibility of burning. The potential and capability of burning increases during extreme heat.
If you are absolutely certain that your vegetable yield will decrease drastically without fertilizer, because you have always fertilized, then at least wait until the temperature drops. But we do not recommend any fertilizer in July and August.
Mulch is always necessary. But during hot dry days and dry nights, it is even more important. It keeps roots cool, seals moisture in and generally buffers the plants. The choice of mulch is determined by the plants, the soil, micro-climate and the philosophy behind gardening. Were you the proverbial fly on the wall during our Master Gardener meetings, you would hear lively discussions about various forms of mulch with advantages and difficulties of each discussed thoroughly. What is important is having mulch.
It's too late for planning strategic shade for this year, but that may be a consideration for next year. If certain plants seem to have too much sun, then look for placement that gives them a partial day of shade. For the plants growing this year, there are options. Some individuals do erect shade structures. Shade cloth is not the only effective barrier. Consider what you have available and how it could be supported. Also explore if removing shade for a portion of each day is feasible or necessary.
Growing natives or drought-tolerant plants means they are more likely to be able to withstand excessive heat. Plants that are not native to this climate will require more vigilance. But most suffer from too much "care" - water, fertilizer or such - rather than neglect. Plants, like people, can withstand much if given the right support.