Pinpoint plant problems

Plant protection helps avoid damage to horticultural crops caused by insects, mites, and diseases. One critical component of plant protection is diagnostics, or assessing what is causing the problem or problems to plants. Plant diagnostics helps determine what is directly and/or indirectly wrong with plants, making it possible to implement corrective measures.

This avoids issues associated with inadequate control or phytotoxicity when using pesticides (insecticides, miticides, and fungicides). When performing plant diagnostics, the first step is to assess patterns of the symptoms expressed by plants. There are three common symptomatic patterns: random, uniform, and clumped. Insect and mite pests, and even diseases, usually cause random or clumped distribution patterns among a crop or individual plant. Environmental, cultural, and/or chemical problems typically result in uniform distribution patterns among a crop. When conducting a diagnosis, it is important to look at the whole plant, including leaves, stems, flowers, and roots.

Biotic and abiotic problems

There are two types of plant problems: biotic and abiotic. Biotic problems are caused by living organisms such as insects, mites, diseases (fungi, bacteria, and viruses), and animals (mice and squirrels). These are considered infectious because they can spread from one plant to the next. Abiotic problems are non-infectious and are disorders caused by environmental parameters, growing medium conditions, and chemical disturbances. This may include growing medium compaction, high or low temperatures, underwatering, air pollution, nutrient deficiencies, and/or chemical injury. Abiotic problems may cause stress, which increases plant susceptibility to biotic disorders. There are various types of stress including shortages or abundance (e.g., moisture, nutrients, and light) and environmental factors (e.g., air pollution, soil pH, oxygen, temperature, and wind). There are many causes of stress such as wounding, poor soil drainage, road salt, compacted soil, chemical damage, root injury, air/water pollutants, temperature extremes, sun scald, fluctuations in soil moisture, improper light, and nutritional deficiencies.

Furthermore, abiotic problems may be acute or chronic. Acute symptoms are expressed almost immediately (e.g., freeze damage) whereas chronic symptoms are cumulative and are expressed over a period of time such as days or weeks (e.g., underfertilizing or drought stress). To make an accurate diagnosis of a plant problem, follow these guidelines:

• Correctly identify the plant as many biotic and abiotic factors are specific to certain plant types.
• Assess possible effects due to environmental factors such as temperature, irrigation, rainfall, and light intensity.
• Determine if any pesticide (insecticide, miticides, and fungicides) or plant growth regulator applications have been made, and be sure to evaluate pesticide records.
• Check leaves, shoots, branches, and roots. Note any discoloration or deformation.
• Check the base of plants (e.g. trunk) for the presence of cankers or wounds.
• Obtain as much background information as possible.

The common symptoms expressed by plants associated with biotic or abiotic factors include yellowing of leaves, wilting, stunted growth, and leaf drop. However, a number of factors may cause each of these common symptoms.

• Leaf yellowing: spider mites, insects, overwatering, nutrient deficiencies, and/or old age.
• Leaf deformation: phloem (e.g., aphids or whiteflies) or xylem (e.g., leafhoppers) feeding insects, virus, underwatering, and/or nutrient deficiencies.
• Leaf drop: disease, improper watering, insufficient light, nitrogen deficiency, and/or air pollution.
• Leaf spot: disease, insects, spider mites, air pollution, and/or nutrient deficiencies.
• Root rot: disease, overwatering, and/or mechanical root injury.

Moreover, factors that may influence the expression of symptoms include host plant, host plant age, time of year, and environmental conditions. Plant problems may be affiliated with insects or mites, diseases, poor growing conditions, or improper nutrition. There may be interactions associated with plant problems. For example, water stress may lead to problems with spider mites, too much fertilizer may result in outbreaks of aphids and spider mites, overwatering may lead to problems with fungus gnat larvae, resulting in more root feeding leading to plant wilting. Some interactions may be more complex. For example, overfertilizing plants may lead to root rot that allows entry of soil-borne plant pathogens (e.g., Pythium spp., and Phytophthora spp.). Consequently, if the plant cannot take up water from the growing medium due to a lack of roots and infection by soil-borne plant pathogens, it may result in moist conditions and fungus gnat problems.

Insects and mites

There are six feeding behaviors associated with insect and mite pests: phloem-feeders (aphids, whiteflies, mealybugs, leafhoppers, and soft scales), xylem-feeders (leafhoppers and spittlebugs), chewers (beetles, caterpillars, sawflies, and fungus gnat larvae), miners (leafminers), chlorophyll-feeders (spider mites and lace bugs), and mesophyll and epidermal fluid-feeders (thrips). These feeding behaviors transcend into specific damage symptoms to plants. Phloem and xylem-feeders, for example, cause plant stunting, wilting, leaf distortion, and leaf yellowing (starting on lower leaves). Chewers remove plant tissues including leaves, stems, flowers, and/or roots. Miners create noticeable serpentine or blotched mines on leaves. Internal feeders (e.g., broad mite) may cause stunted plant growth, small leaves, or symptoms that may resemble nutrient deficiencies or viruses.

There are three insect pests and one mite pest that cause similar damage symptoms to plants; twospotted spider mite (Tetranychus urticae), leafhoppers, thrips, and lace bugs. By understanding the differences among the four it will be easy to distinguish the damage symptoms.

• Twospotted spider mite: foliage appears “speckled” with webbing present. Foliage may also appear bronzed or “dirty” looking; this is dependent on plant type.
• Leafhoppers: foliage has a “speckled” or “bleached” appearance with molting skins of nymphs on leaf undersides.
• Thrips: foliage has a “silvery” appearance with sunken tissues on the leaf underside. Black fecal deposits (“thrips poop”) may be present.
• Lace bugs: foliage has a “bronzed” or “bleached” appearance. Black eggs and fecal deposits along with black nymphs are usually present.

Insect and mite pests cause direct damage as a result of their feeding. However, they may also foment indirect damage through their by-products. This includes honeydew production by phloem-feeding insects such as aphids, whiteflies, mealybugs, and soft scales. Honeydew is a clear sticky liquid that serves as a growing medium for black sooty mold fungi and attracts ants. Whenever ants are observed around plants, this is a good indication that a phloem-feeding insect is present. In addition, aphids leave molting or cast skins on leaves that resemble whiteflies. Caterpillars will deposit fecal matter on plant leaves and stems. All these sources of indirect damage may impact the aesthetic quality of horticultural crops.

If you are still stumped, then submitting samples to a university-based or independent plant diagnostic clinic can help determine the primary problem or problems, and the organism causing the symptoms.

Plant diagnostics is one of the important components of plant protection, and as such it is critical to correctly diagnosis a plant problem(s) in order to implement the appropriate corrective measure. Be cognizant of the different symptomatic patterns (random, clumped, and uniform), and insect and mite pest feeding behaviors (chewing and sucking), and subsequent direct and indirect plant damage that results from insect and mite feeding.

Raymond Cloyd is professor and extension specialist in Horticultural Entomology/Plant Protection at Kansas State University, Department of Entomology; rcloyd@ksu.edu.