Plant Pathology - Abiotic  Distress

A young pepper in distress from cool temperatures.

Recently, we enumerated the common causes of plant diseases in a post entitled  Plant Pathology. Now,  in keeping with the morbid greyness of November, we'll briefly review the character and sources of  "non-disease-related" problems.

Every gardener has encountered a sick looking plant. Some morning you cheerfully get up, grab a cup of coffee and then look at your flower box on the windowsill or maybe step into your sun porch to greet the newest pepper sprouts or marigolds and discover that overnight one, or some, suddenly are, to put it gently, 'failing to thrive.'   

What happened? Who did this? Don't jump to conclusions.

A  first reaction might be to think that some sneaky microbial pest has contaminated your private green domain. The plant  appears "sick," i.e. in some kind of  distress, but it may not be diseased; and the symptom appearing in front of you may be signaling a problem actually orginating in a different part of the plant. The plant's distress might not be caused by some other living thing at all...

Abiotic factors account for a lot more plant problems than you might think. Some experts attribute abiotic issues  as accounting for about 50% of plant problems while a few contend that it might be as much as 80%!  Many abiotic problems are readily remedied (or entirely avoided) by applying simple, good gardening practices - maintaining moisture levels, cultivating regularly to promote aeration and drainage, thinning plants, weeding etc. Some abiotic problems,  such as nutrient deficiencies, are harder to diagnose and require more time to correct; and some, like drought or monsoons, may not have readily available solutions. In my admittedly limited experience, containers, i.e. pots of any size or shape, seem to be prone to abiotic problems: they heat up and dry out fast,  salt residues from old fertilizers or hard water build up and soil fertility declines because soils or nutrients are not  replenished on a regular basis.  So - if you use a lot of containers, it's important that you be alert to the range abiotic problems that might develop.

Types of Abiotic Problems

Abiotic factors fall into several rather distinct, but unsexy,  categories:

Mechanical - These typically are simple breakages or abrasions that are usually caused by wind, hail, dogs, cats and other animals including small children.  [Tiny punctures in leaves or missing leaf tissue, however,  suggests some kind of  pest has been free loading.]

Beans showing some yellowing leaves.
Photo: NYSAES Geneva, NY

Chemical -  Nutritional deficiencies and toxicity frequently show up with disease-like symptoms. Too much of a good thing like fertilizer,  or, as in cases like road salt or pesticides - just a little too much of a bad thing, can cause yellowing (chlorosis) to varying degrees; but yellowing  can also  indicate  viral and bacterial  infestations.

How toxicity shows up depends upon whether a chemical is  absorbed by a plant  or comes into  direct foliar contact with some part of it, e.g. via careless spraying.  In the former case, symptoms may be general in nature, e.g. wilting or poor new shoot development; in the latter, leaf discoloration along with dead spots can rapidly occur.


[See earllier posts:  Macronutrients and Micronutrients about plant nutrition and how to recognize possible deficiences.]

Environmental - Plants, like many of us, typically do not respond well to sudden changes in their environment.  Temperature extremes, lighting,  air quality, oxygen and moisture levels will manifest themselves in various ways such as wilting, leaf drop, lack of flowering, inferior fruit, slow growth etc.  Note: oxygen and moisture levels can be related -- saturated soil that does not drain well will leave roots suffocating and that in turn can cause leaves to wilt, develop marginal browning and eventual dropping.  [And, of course, with respect to roots attacks by parasites and nematodes can also induce similiar symptoms.]  

Factors to Consider in Evaluating Problems

A wilted plant in a dried out pot is clearly distressed and easily resurrected by restoring the soil's moisture level, but a plant showing yellowing leaves as in the bean plants depicted above requires a more  systematic analysis to determine the cause.  Yellowing  leaves can be arise from a variety of factors:  nutrition, environment, parasites, fungi, bacteria,  viruses etc.

So after getting over the shock that your beloved rutabaga looks under the weather, switch on your situational awareness circuit and look at the unhappy specimen in front of you with an open mind and  begin thinking about  at leas some of the following before deciding on a  course of action:

• Does the symptom appear to be confined to just one plant, or are other plants also affected? 

• If more than one plant seems afflicted, is the symptom confined to a small area in your garden involving just a few plants or does it appear in a wider area? [Abiotic problems tend to be uniform in character and, in a garden setting, usually show up aross a wide area and are not limited to just one or a few plants. In fact, different types of plants may be affected.  Keep in mind, however,  that if you see multiple symptoms, there is a good chance that you may be dealing with multiple problems. ]

• If lately  you've been suspicious about the health of a plant, does the symptom seem to be changing over time?  

• Is the symptom spreading on this plant or are other plants showing lesser or greater degrees of the symptom?  [If yes, this implies a biotic cause.]

• Is just a specific variety of plant affected? [If yes, this also implies a biotic cause.]

• What part of the plant is affected?  Look at the whole plant -  check its stems, leaves, roots, blossoms, fruit. 

• Where is  the symptom located:  low or high on the stem.  

• Does the symptom appear to be confined to leaves, stems, blossoms or just to the tissue between leaf veins? 

• Consider the plant's setting. 

• Is the soil compacted,  loose, wet or dry;

• How much sunlight does the plant really get;

• Is the plant crowded? [Did you forget about thinning to the recommended spacing...]

• Has the weather been hot or cool?

• Are leaves or fruit touching the soil or other plants?

• If you are thinking that an insect or similar  pest is responsible, look on the underside of leaves and on stems for egg or spore clusters or  frass.  Snails (mullusks) may have left slime trails in their  retreat to a secure, remote daytime location.

• Finally, do you remember anything about  last year's garden problems?   Is what you are seeing  similar to anything that occured last year? Both abiotic and biotic problems tend to recur especially in small gardens.   

Be Your Own Plant Dr.

For gardener's who would like to minimize their reliance on the Internet and use a book to solve their plants' problems, I recommend "What's Wrong With My Plant (and how do I fix it?"   by David Deardorff and Kathryn Wadsworth (Timber Press, 2009).  The book served as one of the texts for my Master Gardener training program. The first half follows a decision tree format employing exceptionally  clear illustrations  -   you start with a symptom and then choose the next refining step from a choice of equally clear illustrations with accompanying text.

The second half of the  book discusses the diseases and disorders in detail as well as organic remedy options.  Deardorff is a plant pathologist at the U. Hawaii and serves as a consultant to the U. of Washington's Extension Service. The co-author is a professional photographer.  Both hardcover and paperback editions are available.


And remember:

"My green thumb came only as a result of the mistakes I made while learning 

to see things from the plant's point of view." - H. Fred Ale