Crop rotation reduces disease and fertilizer problems

A YouTube video of a home gardener caught my attention recently. The gardener said that crop rotation (putting vegetables in different spots each time you plant to reduce disease and insect problems) wasn’t important for home gardeners. He said it was meant for commercial growers.

I could not disagree more. Crop rotation has been around for more than 150 years as a good management technique for reducing disease and fertilizer problems, whether you grow a small number of plants or large numbers. I have seen this technique violated by small-scale growers in other countries with total disease devastation to tomatoes, peppers and cucumbers.

This technique switches the growth of vegetables to new locations every three to five years. It’s best to understand the families that vegetables belong to, but if you don’t, then always grow vegetables that produce root crops (carrots, beets) in spots that previously had vegetables from flowers (tomato, pepper, eggplant) and follow that by growing leafy vegetables (lettuce, spinach, chard).

If you are using a single raised bed, then simply grow these vegetables in new locations in the raised bed so they aren’t grown in the same location. Don’t grow vegetables in the same location for three to five years.

An alternative method is to loosen the soil, lightly water it and then cover it tightly with clear plastic for three or four hot days in full sun, letting it bake. This method, called soil solarization, controls several plant diseases inhabiting in the soil, as well as many insect pests and weed seeds.

This YouTube gardener may have success now without using crop rotation, but he will learn this lesson the hard way in future years. Probably the worst part is that this propaganda will circulate on YouTube for many years to come and will lead home gardeners down a very dangerous and uneducated horticultural path.

Q: We have an apricot tree about 5 years old. It’s always performed well, but this year it leafed out beautifully then last week started losing its healthy green leaves on one side. Apricots are on the tree, but not developed yet. Now half the branches on that one side are leafless.

A: I looked at the picture and it looks like a watering problem — not enough water getting to the upper limbs and causing drought and leaf drop. Drought occurs first on the hottest side, the sides facing the sun. If you remember, we had a hot spell just before the weather cooled off again.

Go to your tree and bend the smallest branches to see if they are still supple. For most fruit trees, if this problem is temporary, they will be supple and bend easily without breaking. If this is the case, expect the tree to produce more leaves and branches after a couple of weeks if it is given water.

Following the KISS rationale, we can eliminate the less likely disease problems, a weed killer spray drifting toward the tree, fertilizer misapplications and the like.

Now, the cause of the drought. Water shortage to these limbs could result from not enough water applied to the tree or in the right areas, or damage to the trunk or limbs by boring insects tunneling through the trunk and limbs and interrupting the flow of water. If sap is not seen on the trunk or branches or the bark is not peeling off, then we can also probably eliminate borers.

Leaf browning and dropping from borer damage usually happens around June or July when it gets hot. But inspect the trunk and limbs anyway for signs of borers.

That leaves us with irrigation, which is the most likely reason particularly because of the unusual weather. A 5-year-old tree should be pretty big, so its water needs are also large unless you have purposely kept it smaller through pruning.

A 5-year-old tree probably needs around 25 to 30 gallons of water each time it is irrigated. This water should be applied to as much area under the canopy as possible.

The minimum number of emitters needed for a 5-year-old tree is four, each located about 12 to 18 inches from the trunk in a square pattern surrounding it. Six emitters arranged in a circle 18 inches from the trunk would be even better.

Apply the water long enough so it penetrates to about 18 inches deep. I don’t know how many minutes this would be because each irrigation system is different. But keep the number of minutes the same as before and increase the size of the drip emitters instead if the water is not deep enough.

To judge if you applied enough water, use a long steel rod, like a 3-foot length of rebar, and push it into the wet soil after an irrigation. Do this in about three or four locations to get an average irrigation depth.

In June, water three times a week when it starts to get hot. If you aren’t sure when to irrigate, use a soil moisture meter like those for houseplants to measure if the soil is dry enough to water again. Don’t trust the surface of the soil to tell you. Push the tip in the soil about 4 to 6 inches deep to get a measurement.

Q: I have six tomato plants grown from seed, all indeterminate. They were growing in 2-year-old rich soil mix containing compost that produced a lot of tomatoes last year. The leaves of the tomato plants developed OK, but then they got spotty and dried up. No blossoms at all. The leafy vegetables grew well, but the older leaves at the bottom of lettuce also developed black spots, became yellow and died.

A: Tomato diseases are tough to diagnose with pictures, but I think this might be a bacterial leaf spot. I got suspicious when you said you started these plants from seed. Then I considered the wet, cool weather, which is perfect for this disease to develop, and concluded it was probably bacterial leaf spot.

Google some pictures of this disease on tomato and lettuce and see if you think it looks like the same and let me know. Leaf spot diseases such as bacterial leaf spot can occur on lettuce as well.

Leaf spot diseases can infect the plant from seed contaminated with this disease agent. Most seed will have this disease present, so applying a hot water or chlorine seed disinfection helps reduce this disease possibility. Methods for doing this at home can be found on the internet, or I can forward instructions to you.

Cool and wet weather, perfect weather for this disease, promotes bacterial leaf spot if it’s present. This disease can become a problem if the growing area has not been cleaned up and old fruit and fines are present.

Also, don’t grow vegetables in the same location year after year (implement crop rotation) to reduce the possibility of this disease, and several others, from occurring. Crop rotation can be as simple as not growing the same vegetable in the same location in a raised bed year after year.

Indeterminate tomatoes have vines that keep getting longer and longer, while determinate tomatoes produce short vines and flower sooner. For earlier production of tomatoes, choose determinate tomatoes rather than indeterminate ones.

Determinate tomatoes are better for most home gardens in the hot desert anyway because you want to produce tomatoes as quickly as possible. Hot weather can come quickly, and air temperatures above 95 F can stop tomatoes from forming from the flowers.

If you select indeterminate tomatoes and then fertilize them with high-nitrogen fertilizers, the plants will grow beautifully but produce few flowers until they get older. When growing indeterminate tomatoes, don’t fertilize plants after planting until they begin to bear fruit.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.