Diagnosing Landscape Plant Problems

Bonnie Nowicki

Most of the following information is from the University of Arizona Desert Landscaping manual I refer to for some of my articles. Let’s start by thinking about picking the right plant for the right place when we are purchasing plants. Select those that are adapted to an arid desert environment, and plant them where they’ll get enough sunlight or shade and have adequate room to grow.

Three elements make up the “Plant Disease Triangle”: The actual plant, the environment, and the pathogen. Each plant specimen has strengths, weaknesses, and needs. They become stressed if their needs are not met. Too much or too little water, the level of sunlight, or proper temperature could factor into their stress. The environment is the place where the plant grows, so soil composition is important. Soil plant nutrients fall into two categories. Both macronutrients and micronutrients are naturally obtained by roots from the soil. Macronutrient elements needed in large amounts include nitrogen, potassium, sulfur, calcium, etc. Micronutrients are those elements needed in small amounts, including iron, zinc, and copper. If the soil isn’t sufficiently moist to allow the roots to take up the nutrients, correcting improper watering methods could eliminate deficiency symptoms. Too much fertilizer can be toxic to plants. Salt burn on foliage or browning on the leaf edges are symptoms. In Arizona soils, nitrogen, phosphorus, and iron are commonly lacking nutrients. Purchasing a moisture, light, and pH combo meter would be helpful to test your soil. I use my dependable meter all the time, especially for moisture testing.

Now for the pathogen part of the disease triangle, which is an agent able to cause disease. It can be a living organism, such as insects, parasites, bacteria, etc., or a non-living pathogen like chemical injury, sunburn, nutrient deficiency, or pollution. Non-living pathogens cause a sudden onset of symptoms, such as distorted leaf shapes, which means a lack of calcium, or yellowing of older leaves, indicating a nitrogen deficiency. Living pathogens leave evidence of their presence, like chewed or ragged leaf edges or holes, fungus, or sticky secretions.

Using these three elements is the best method of managing and diagnosing the problem. You can modify the environment, remove the pathogen, or replace the plant. Your experience, good judgment, and a bit of luck can help this process. If you are unable to determine the cause by yourself, stop by the U of A Extension Center, 10 minutes from Quail Creek, at 530 E. Whitehouse Canyon Road. They’ve got the resources and ability to help solve your landscape plant problems. Happy gardening!