There are many reasons why it is good to live in a desert with low humidity. One of them is fewer disease problems. Desert climates have notoriously low humidity, especially when it gets warmer, so we have fewer plant diseases and are less worried than in places like Seattle or Tampa, Florida. But plants that grow in the desert get plant diseases.
Plant diseases are divided into two types: those caused by living pathogens such as bacteria, fungi and viruses (biotic) and those caused by non-living pathogens (abiotic).
Cures for biotic diseases, when properly identified, can be found by Googling or watching YouTube videos. The healing of these biotic diseases can be approached nationally or internationally. But abiotic plant diseases promoted by living in our desert climate – strong sunlight, poor soil drainage, plant selection – are desert conditions and are the most troublesome for the people who move here.
The Disease Triangle is a powerful concept for combating plant diseases of all kinds. It is believed that three things are necessary for disease of all kinds to break out: the presence of a pathogen, the susceptibility of certain plants to disease, and the right environment.
Remove any of the three parts of the disease triangle and the disease will stop.
Preventing diseases takes time and energy to implement. Disinfecting mowers and hand tools helps remove biotic pathogens. Likewise the control of weeds and insects.
Choosing varieties or plant species that are better suited to dealing with heat, cold or low humidity addresses the second side of the triangle. Choosing the location for a raised bed or planting a type of tree or shrub that is best suited for a particular home microclimate addresses the third. Not doing these things costs money instead of spending time and energy on prevention.
Nevertheless, biotic diseases do occur. Fire blight and koryneum rot are two biotic diseases that are minimized by choosing the right species or variety of fruit trees. Summer rot on high fescue lawns is minimized by cleaning the mower and managing the lawn differently.
Before buying any disease control chemical, consider trying the triangle approach to disease prevention.
Q: My tomato plants are growing well, but I wonder if the leaves could tell me if I am missing something. Some leaves are very flat and what I would consider normal. Other leaves are curled around the edges. I thought of aphids at first, but they don’t seem to be on the leaves.
A: What is normal? Knowing how normal looks comes from seeing many different types of tomatoes, either from years of gardening experience or looking at lots of tomato leaves.
They didn’t tell me what type of tomato to look at or whether to look between different varieties. Some tomato varieties have relatively flat leaves and other varieties have curled leaves.
Curling leaves can resemble an aphid infestation. Seasoned gardeners know the difference between bumpy but flat San Marzano leaf varieties and the crinkled Early Girl leaf variety. Sometimes, curling leaves can result from the heat. This is where experience in gardening becomes important.
If my tomato plants appear healthy and fruiting as they should at this time of year, I don’t worry about whether or not they will curl. I worry if they are discolored (brown), damaged, or not producing as they should. It’s a lot of detective work.
I don’t like seeing very dark green leaves on plants preparing to flower. A dark green leaf color when trying to bloom can mean access to very rich soils or an overdose of nitrogen fertilizer.
If the plants are very young, dark green is fine. But if you want them to bear fruit later, plants receiving too much nitrogen – dark green leaves – can delay flowering.
With some varieties this leads to “all wine and no fruit”. Tomatoes should be hungry when they start on the fruit. Once they start producing, feed them.
Q: Last fall, I bought a 50 pound sack of large alfalfa pellets for $ 11. My first idea was to use it as a nitrogen source in my compost heap. Another option is to add it to my raised beds as is and let it decompose. The third idea was to make alfalfa flour tea by sitting in a larger bucket of water and allowing it to ferment for several days to a week and then pour it with it. What’s the best?
A: I have never worked with alfalfa pellets so I have no experience other than using it as animal feed. Most compost, if obtained from animals, is high in phosphorus and is high in microorganisms. Personally, I think it is wasted as a compost additive because there is a lot of nitrogen-rich kitchen waste to be composted.
One thing I like about alfalfa pellets is their high potassium level. Potassium is hard to come by in organic form.
I like that they use molasses as a binder for the pellets, which probably helps with feeding too. Molasses is part of the Korean natural growing method used in Asia. The University of Hawaii speaks of a technique for increasing the number of microorganisms in the soil. In my opinion, the main benefit of natural farming is to stimulate microbial activity.
I like your ideas of using it as a floor application. I would take a close look at Korean natural growing techniques for reducing fertilizer needs and discussed by the University of Hawaii.
Alfalfa pellets keep dry for a long time, so I would use them occasionally if needed to boost production. I only like it as a soil amendment supplemented with molasses. I don’t think natural farming has been tried or popularized as much on desert soils as it was in Asia.
Q: Have you had any experience using potassium bicarbonate in a solution for both treating and preventing powdery mildew?
A: I have no experience of using potassium bicarbonate to control powdery mildew. I suggest using the disease triangle and preventing this disease by adding more sunlight, reducing water splashes on the leaves, and increasing dry air movement and drying of the leaves by pruning and removing the leaves (environmental change).
Different powdery mildew diseases are specific to each plant, but overall this disease requires low humidity compared to many other fungal diseases. Powdery mildew in grapes is prevented primarily by defoliation and improving air circulation. Fungicides are occasionally needed when outbreaks are triggered by spring rains.
Q: I have a 3 year old pomegranate bush with stems that are now dying as the weather has gotten hot. What do you recommend to do?
A: A common disease of fruit trees in more humid climates is root or collar rot, which is caused by a fungal soil disease organism that likes wet soil called Phytophthora. This disease can become a problem with pomegranate when the soil is kept moist, when mulch on the soil touches the stems of young plants and when there is poor drainage. It is most common on avocado trees and other fruit trees when they are young.
The soil has to dry on the surface and anything that touches the stems wet is raked back. Dry soils contain this disease, but once the soils get wet, this disease organism begins to thrive.
It attacks the parts of the shrub that penetrate the soil, suffocate the tips and cause them to die. It’s a bigger problem when the temperatures are warm or hot because that’s when the disease organism thrives and plants need the most water.
Preventing this disease is much easier and cheaper than using fungicides to control it. But fungicides like copper compounds may be needed to prevent exacerbation.
Let the soil dry out and watch out for moisture, especially the surface. Clear the area around these stems of weeds and any mulch that touches the stems.
Use of fungicides to control Phytophthora collar rot may be recommended until irrigation and soil moisture are under control, the disease clears, and the soil is dry again. If the disease is not stopped, it can spread to the roots and cause the entire plant to die.
Bob Morris is a horticultural expert and Professor Emeritus from the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.
