Las Vegas’ dry desert reduces disease problems in plants

There are many reasons why living in a desert with low humidity is a good thing. One is less disease problems. Desert climates are notoriously low in humidity, especially as the day gets warmer, so we enjoy plant diseases less and worry about them less than in places like Seattle or Tampa, Florida. But the plants that grow in the desert contract plant diseases.

Plant diseases are classified into two types: those caused by living agents such as bacteria, fungi and viruses (biotics) and those caused by non-living agents (abiotics).

Cures for biotic diseases, if identified correctly, can be found by Googling or watching YouTube videos. Curing these biotic diseases can be approached nationally or internationally. But abiotic plant diseases, those encouraged by life in our desert climate – strong sunshine, poor soil drainage, plant breeding – are desert conditions and cause the most problems for people who settle there.

The disease triangle is a powerful concept for controlling plant diseases of all types. He assumes that three things are necessary for diseases of all types to begin: the presence of a pathogen, the susceptibility of certain plants to diseases, and a suitable environment.

Remove one of the three parts of the disease triangle and the disease stops.

Preventing diseases from starting costs time and energy to implement. Disinfecting mowers and hand tools helps eliminate biotic pathogens. The same goes for weed and insect control.

Selecting which varieties or types of plants are best equipped to handle heat, cold, or low humidity addresses the second side of the triangle. Choosing where to place a raised bed or plant a type of tree or shrub best suited to a particular home microclimate addresses the third. Not doing these things costs money instead of the time and energy spent on prevention.

Yet biotic diseases do occur. Fire blight and coryneum blight are two biotic diseases that are minimized by selecting the right type or variety of fruit trees. The summer blight of tall fescue lawns is minimized by cleaning the mowers and managing the lawn differently.

Before buying a chemical to control disease after it appears, try the triangular approach to disease prevention.

Q: My tomato plants are growing well, but I’m wondering if the leaves could tell me if something is missing. Some sheets are very flat and what I consider normal. The other leaves are rolled up around the edges. I first thought of aphids, but they don’t seem to be on the leaves.

A: What is normal? Knowing what normal looks like comes from seeing a lot of different types of tomatoes, either from years of gardening experience or looking at a lot of tomato leaves.

You did not tell me which variety of tomato you are looking for or if you are looking among different varieties. Some varieties of tomatoes have relatively flat leaves and other varieties have rolled leaves.

The curly leaves can look like aphid infestations. Experienced gardeners know the difference between the bumpy but flat San Marzano leaf varieties and the rolled up Early Girl leaf variety. Sometimes curling sheets can be caused by heat. This is where gardening experience becomes important.

If my tomato plants are looking healthy and producing fruit like they should at this time of year, I don’t care whether they are curly or not. I start to worry when they are discolored (brown), damaged, or not producing as they should. It’s a lot of detective work.

I don’t like to see very dark green leaves in plants that are getting ready to flower. The dark green color of the leaves when trying to flower can signify access to very rich soils or an excessive application of nitrogen fertilizers.

When the plants are very young, dark green is suitable. But when you want them to produce fruit later, plants fed too much nitrogen (dark green leaves) can delay flowering.

In some varieties this translates to “all vines and no fruit”. The tomatoes should be hungry when they start to fruit. Once they start producing, go ahead and feed them.

Q: Last fall I bought a 50 pound bag of alfalfa coarse pellets for $ 11. My first idea was to use it as a source of nitrogen in my compost pile. Another option is to add it to my raised beds as is and let it decompose. The third idea was to make alfalfa meal tea by letting it sit in a larger bucket of water and ferment for several days to a week, then basting with it. What is the best ?

A: I have never worked with alfalfa pellets, so I have no experience other than using it as animal feed. Most composts are high in phosphorus if they come from animals and are high in microorganisms. Personally, I think it is wasted as a compost amendment because there are a lot of nitrogen-rich kitchen scraps available for composting.

One thing I love about alfalfa pellets is their high potassium value. Potassium is difficult to obtain in an organic form.

I like that they use molasses as a binder for the pellets, which probably helps with feeding as well. Molasses is part of the Korean natural farming method used in Asia. The University of Hawaii talks about it as a technique used to increase the number of microorganisms in the soil. In my opinion, the main advantage of natural agriculture is the stimulation of microbial activity.

I like your ideas on using it as a ground application. I would take a close look at Korean natural agricultural techniques to reduce the need for fertilizer and discussed by the University of Hawaii.

Alfalfa pellets will keep dry for a long time, so I would use them to occasionally increase production when needed. I just like it as a soil amendment supplementing it with molasses. I don’t think natural agriculture has been tried or popularized in desert soils as much as it has been popularized in Asia.

Q: Have you ever used potassium bicarbonate solution for both the treatment and preventative control of powdery mildew?

A: I have no experience with potassium bicarbonate for powdery mildew. I suggest using the disease triangle and preventing this disease by adding more sunlight, reducing water splashing on the leaves, and increasing the movement of dry air and drying the leaves by pruning. and leaf removal (environmental modification).

Different powdery mildew diseases are specific to each plant, but collectively this disease requires low humidity compared to many other fungal diseases. Grape powdery mildew is prevented primarily by leaf stripping and improving air circulation. Sometimes fungicides are needed when outbreaks are triggered by spring rains.

Q: I have a 3 year old pomegranate shrub with stems that are dying now that the weather has turned hot. What do you recommend doing?

A: A common disease of fruit trees in more humid climates is root or crown rot caused by a soil fungal organism that likes moist soils called Phytophthora. This disease can be a problem in pomegranate if the soil is kept moist, if the mulch on the ground touches the stems of young plants, and if there is poor water drainage. It is most common in avocado and other fruit trees when they are young.

The soil should have a chance to dry out on the surface, and anything that touches the wet stems is raked. Dry soils contain this disease, but as soon as the soils become wet, this pathogenic organism begins to thrive.

It attacks the parts of the shrub that penetrate the soil, smothering the tops and causing their death. This is more of a problem when temperatures are hot or warm, as this is when the pathogen thrives and plants need water the most.

Preventing this disease is much easier and cheaper than applying fungicides to control it. But fungicides such as copper compounds may be needed to keep it from getting worse.

Allow the soil to dry and watch it for moisture, especially the surface. Clear the area around these stems of weeds and any mulch that touches the stems.

Applications of fungicides that control Phytophthora crown rot may be recommended until irrigation and soil moisture are under control, the disease stops and the soil is dry again. If the disease is not stopped, it can spread to the roots and cause the death of the whole plant.

Bob Morris is a horticultural expert and Professor Emeritus at the University of Nevada, Las Vegas. Visit his blog at Send your questions to

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