Diagnosing Plant Problems 



Cheryl A. Smith 



The diagnosis of plant problems can be a challeng- 

 ing task, requiring a basic knowledge of plant cui 

 ture and physiology and environmental influences 

 on plant health, as well as the ability to identify the 

 possible causes Developing a solution or remedy to 

 the problem depends upon a proper diagnosis. The 

 process of diagnosing plant problems generally consists 

 of (1) recognition or perception of a problem, and |2| 

 determination of the cause (or causes). 



Symptom recognition 



The first step in diagnosing a plant health problem is 

 the recognition of symptoms A symptom is any visible, 

 'abnormal' condition of a plant caused by biotic agents 

 (living organisms such as diseases and insects) or abi- 

 otic agents (non-living or environmental factors). Before 

 symptoms can be recognized however, the horticulturist 

 must be familiar with the characteristics of the 'normal' 

 plant. What may appear to be a symptom may actually 

 be 'normal' for a particular variety. Common symptoms 

 of plant diseases and disorders include leaf spots and 

 blotches, yellowing (chlorosis), marginal browning (ne- 

 crosis) of leaves, stunting, dieback, distorted growth, 

 galls, leaf drop, stem cankers, wilt, and root rot. 



Is there a pattern to the symptoms? 



Once symptoms are noted, a general assessment 

 should be made of the affected plant(s) and nearby 

 healthy plants. A series of questions may be helpful in 

 assessing the problem. Is more than one plant affected 

 and is more than one plant genera or species affected? 

 Where on the plant(s) did the symptoms first appear? 

 Is the problem limited to the interior or exterior por- 

 tions of the plant (or planting)? Are the symptoms very 

 localized or are they widespread? Are several types of 

 symptoms present? After making a general assessment, 

 take a closer look at the symptoms. It is often helpful 

 to determine the shape and pattern of leaf spots. Do 

 the spots have concentric rings or a zonate appearance? 

 Are the spots round or angular? Do the spots/lesions 

 appear to be limited by the veins? Be sure to check 

 the roots whenever possible. Symptoms of nutrient de- 

 ficiency, marginal necrosis of leaves, and wilting are of- 

 ten symptoms associated with root rots. 

 If more than one plant genera is affected, the cause is 

 usually due to an abiotic agent. If the symptoms are 

 limited to a single species, the problem is more likely 

 caused by a living agent. Keep in mind however, that a 

 particular species of plant may be more or less sensi- 



tive to chemical problems (micro-nutrient levels, 

 soluble salts, pesticides). Symptoms appearing only on 

 one side of a plant or planting or in a repeated pattern 

 such as every two plants or every other row, are most 

 likely caused by abiotic agents. Symptoms caused by 

 biotic agents are more likely to be random in occur- 

 rence or pattern. 



Are signs visible? 



Signs are the actual visible evidence of pathogens and/ 

 or insect pests. A I5x or 20x hand lens is helpful for 

 viewing fungal structures and insects or mites Examine 

 the symptomatic plants for fungal fruiting bodies (black 

 or brown pinpoint-size structures) or fungal growth 

 (molds or strand-like growth). Signs of insects include 

 the insect itself (and all its life stages), cast exoskel- 

 etons (skins), webbing, or droppings (frass or honey- 

 dew). Finding evidence of a pathogen or insect may not 

 lead directly to the cause of the problem, however. 

 Sometimes abiotic factors can weaken a plant and pre- 

 dispose it to attack by pathogens and/or insects. For 

 example, high soluble salts can predispose plants to 

 root rot fungi and drought often predisposes trees to 

 fungal tip blights or insect attacks. 



How quickly did the symptoms appear? 



Another aspect to consider is the time frame during 

 which symptoms appeared. A record of the environmen- 

 tal conditions during that time period may also be use- 

 ful in determining the causes of the problem. Once 

 again, a series of questions may be helpful. How long 

 has the problem existed (when did the symptoms first 

 occur)? This is often a difficult question, as many prob- 

 lems seem to appear overnight. Symptoms caused by 

 most biotic agents take several days or longer to de- 

 velop. As always, there are a few exceptions: bacterial 

 diseases can develop and spread rapidly and pythium 

 blight of turfgrasses can spread significantly in 24 hours. 

 The sudden appearance (one to two days) of symp- 

 toms, however, is usually caused by abiotic agents. 



Record-keeping 



Detailed record-keeping of general plant health at regu- 

 lar intervals (daily, weekly, etc.) will help trace the ap- 

 pearance of symptoms. Records are also helpful when 

 trying to determine if particular management or cultural 

 practices may have caused the symptoms. It is impor- 

 tant to keep records of fertilizer and pesticide applica- 

 tions (date and rate), and watering practices, as well as 

 the introduction of new plant material (they may be the 



DECEMBER 1997 & fANUARY 1998 



