Late blight is a fungal disease that can devastate a tomato or potato field within two or three weeks if it is not properly controlled.

Citrus canker is caused by a bacterial pathogen called Xanthomonas axonopodis pv. citri, and is unfortunately well known in Florida.

The first step in diagnosing a disorder is to look for symptoms, which are changes in a plant’s normal appearance, and possibly signs, which are evidence of the actual causal agent. Symptoms include: galls, leaf spots, wilts, and viral spots, or unusual leaf shapes or colors. Signs include: fungal spores, bacterial ooze, or the insect, mite, or other animal which is causing the problem. In some cases, the disorder is caused by a living organism such as an insect or insect-like pest or by a plant disease. In other cases, the cause is abiotic, or not caused by a living organism.

Living Organisms

Do some detective work. Start by determining that an actual problem exists. Look for patterns: in the whole field or grove, on an individual plant, or on a specific plant part; determine how long it has taken for the pattern to develop; if you are not the grower, ask him or her questions, and finally, synthesize the information.

Symptoms of fungal diseases can include spots, blights, galls, rots, and wilts; while fungal structures are signs. Symptoms of bacterial diseases can include galls, leaf scales, cankers, stem or leaf rots, wilts, leaf spots and blights, scabs, soft decay of fruit, roots, and stems, and sour-smelling roots, stems, or fruit; while bacterial ooze is a sign. Symptoms of viral diseases include curling leaves, stunted plants, wilting, different colored spots on leaves and fruit, misshapen or yellowing leaves, and a blotchy or mosaic pattern of light and dark color in leaves. Some viruses produce symptoms that resemble damage from specific herbicides. Wilt diseases can be caused by either fungi or bacteria. A typical symptom is browning of the vascular system, which can be seen if one makes a vertical slice through the stem or branch.

Common symptoms of insect damage include: defoliation or holes in leaves and stems caused by chewing insects, stippling caused by insects with piercing-sucking mouthparts, dieback and distortion caused by feeding, frass from caterpillars and thrips, and silk-like webs from spider mites. The actual insect or mite is a sign.

Abiotic Disorders

Many times, the effects of abiotic disorders are cumulative and subtle, such as a prolonged drought or an acid-loving plant growing in alkaline soil, which makes them difficult to diagnose. On the other hand, it may be fairly easy to diagnose herbicide or other chemical damage or freeze or frost injury since these disorders are visible immediately or shortly after the damaging event has taken place.

While plant diseases and insects are caused by or are living organisms, abiotic disorders are caused by things that are not alive. They can be classified based on the causal factor or symptom. Some are caused by environmental conditions (climate and weather), and others by applications of chemicals such as fertilizers, pesticides, and especially herbicides.

Detective work is required for abiotic disorders, too. Diagnosis begins with physically examining an individual plant or a group of plants that are growing near each other. Abiotic disorders may affect only certain parts of a plant, so you must be able to recognize and accurately describe where specific signs and symptoms are found. Some abiotic disorders, such as lightning strikes, may affect a few plants that are next to each other. Herbicide or other chemical injury, on the other hand, usually occurs uniformly across a wider area. Most abiotic problems require you to consider factors affecting both the plant and the site and to determine probable cause through a process of elimination.

Environmental conditions. Injury can occur when temperatures are either above freezing, but low enough to cause damage, or when they fall below freezing. Symptoms of chilling injury include wilting, desiccation, and possibly physical distortion of plant parts.

Freezing injury is caused by temperatures at or below freezing. Typical signs include discoloration of plant tissue. Sometimes it is limited to flower buds and some shoot dieback, but at other times the entire plant may die outright or suffer so much damage that it becomes unprofitable to maintain. In some cases, freeze damage on annual plants such as vegetables can resemble damage from a contact herbicide such as paraquat.

Other environmental factors include: drought, heat, flooding, excessive wind, lightning, and hail. Wilting is an initial symptom of both drought and flooding. It can resemble wilts caused by plant diseases, but plants may recover if conditions improve quickly enough.

Chemical injury. Herbicide injury symptoms vary with the family of herbicide which was used. Damage is uniform across the treated area, with all susceptible plants being affected. The growth regulator herbicides such as the phenoxy herbicides (2,4-D and triclopyr) cause strapping and other distortions in leaves and may cause parthenocarpic fruit (no seeds) in vegetables such as tomato and eggplant. Photosynthetic inhibitors such as the triazines interrupt normal photosynthetic processes. Injury symptoms occur on older leaves first and include leaf yellowing and interveinal chlorosis. Plant enzyme inhibitors, such as glyphosate, cause injury symptoms that include leaf yellowing, browning, and distorted tips. These symptoms do not appear immediately because this type of herbicide must be translocated throughout the plant.

The Bottom Line

In the end, you must be able to determine whether the disorder is caused by a living organism, which you may be able to treat with a pesticide, or by something abiotic, which may be more difficult to correct.