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

ROOT CONCERNS -  A HORTICULTURAL NEWSLETTER

Root Concerns, a gardening newsletter produced by the Cornell Cooperative Extensions of Rensselaer, Albany, and Schenectady Counties, is distributed free via email. Issues usually cover a potpourri of topics from A (as in A Horizon, the top level of soil) to Z (as in zygopetalum, a type of orchid).

The current issue of Root Concerns includes articles about witchhazel, maple syrup, oak wilt, kale, and gardens in New Zealand and Alaska. To view the current issue click on this link,  March 2018.

To subscribe to the newsletter just register with Marcie Mohnoutka via  mmp74@cornell.edu at the  Rensselaer County Cooperative Extension and request that your email address be added to the distribution list.

You can also peruse the archive of Root Concerns' back issues from a link in the  Related Resources list located on the  right side of this blog page.

Plant Nutrition : Macronutrients 

While we wait for the snow to melt and our gardens to dry out, let's review what plants need and   what fertilizers provide.   Also, if you haven't had your garden soil recently tested and think your vegetables are not up to snuff, we'll identify some of the major "aha" nutritional deficiencies you can look for....   You'll just have to try to remember all this once the growing season starts...

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Every bear in the woods probably knows that plants use the three elements, Hydrogen, Oxygen and Carbon. Plants obtain these three  from  air and water.  But there are 14 other elements that are vital for plant nutrition. Of these, six are identified as "macronutrients" with the remaining 8 elements characterized as "micronutrients" or "trace elements."  Plants actually use other elements as well, but these 14 are needed by all plants. We'll cover micronutrients later  in  a separate post. 

Macronutrients

 Lack of Ca. Source: Cornell Extension

Calcium (Ca)  - Calcium is generally available in soils and is taken into plants through evapotranspiration (water siphoning from the roots through the leaves to the atmosphere). It is the key ingredient  in plant  cell walls. During times of drought (and/or a gardener's under watering) or high humidity, plants do not move enough water through their tissues to supply cell needs resulting  in calcium deficiency.  This is important to note because calcium may not necessarily be deficient in the soil. In either case, this deficiency dramatically manifests itself in tomatoes and peppers, for example,  as blossom end rot. An over abundance of calcium is more subtle to detect because  too much calcium can interfere with the take up of other elements whose deficiencies may exhibit markedly different symptoms.

Chlorosis.  Source: Cornell Extension

Magnesium (Mg)  - Also typically available in soils, especially in clays.  It is incorporated into chlorophyll and plant enzymes.  Its deficiency appears initially as raised areas of  light yellow in mature leaves.  Later, these areas become more pronounce and eventually turn into dead leaf areas (chlorosis). 

Nitrogen (N)   - Nitrogen is utilized by plants in several forms,  but let's skip  over the complex chemistry of the Nitrogen Cycle and just say  that  plants  absorb  these  compounds  incorporating  much of it into   their leaves and the enzymes essential for photosynthesis.  A deficiency will first appear as a paling of mature leaves, then evolve to  yellowing before ending in chlorosis (dead leaves).  Although  nitrogen is essential for plant growth, too much will also cause chlorosis in leaves.  

[n.b. Fertilizers meant for house plants  are typically loaded with nitrogen; and if you use them on vegetables, you will get lots of vegetative growth and less fruit.]

Excess Phosphorus.  Source: U Maryland Extension

Phosphorus (P)   - Phosphorus is used in cell membranes and serves as the primary carrier of energy throughout plant cells.  It is incorporated into DNA and RNA as well as  regulating enzyme activity. It generally occurs naturally in soils, but its availability to plants depends upon soil pH.  A deficiency shows up as dark green or  purplish tints in tomato leaves. Eventually, flowers and fruit do not develop.

 

Potassium deficiency.  Source: Yara.us 

Potassium (K)   - Plants need potassium almost as much as they   need nitrogen, and   potassium also abets nitrogen take up. It maintains various metabolic activities by regulating water within the plant. Potassium deficiency may first appear as yellowing of older leaves before they turn brown. (This may also be called leaf scorch or leaf burn, but other conditions such as exposure to salts, insecticides or too much fertilizer might also cause it.)   The taste and appearance of fruit can also be affected.  

Sulfur deficiency.  Source: haifa-group.com

Sulfur (S)  - Sulfur is utilized in amino acids, proteins, vitamins  and other compounds, e.g. sulfur gives onions and mustard their distinct flavors.  Although sulfur also occurs naturally in some soils, its concentration is relatively low compared with other nutrients and  tends to be  especially low  in sandy soils. Much of the sulphur that is found in soils derives  from the decomposition of  organic materials, e.g. compost.  [This is another  good reason for you to start composting.] A very small amount of  sulphur can also  come from the atmosphere.  Its deficiency appears as yellowing of younger leaves and woody looking stems.

Nutrients and Fertilizers

From what I have observed of neighbors and friends who  try  container gardening, nutrient deficiencies  can show up  relatively quickly because the gardeners do not replenish  the soils from year to year, fail to fertilize during the growing season and/or do not maintain adequate moisture levels. For gardeners, like me, who dig up their front or backyards, deficiencies may take many growing seasons to appear because we are accessing  a greater reservoir of nutrients. Maintaining a consistent moisture level (apart from timely cultivating) is probably our major challenge!

Soils typically have all of the above macronutrients to some extent because these elements  originate from mineralogical sources. However, large percentages of these elements  frequently  are chemically bonded with other compounds in the soil  and rendered  unavailable to plants.  Over time  the usable forms of these  elements   decline as  crops are harvested, remaining plant stock  removed (i.e. diseased plants pulled up and stuffed into lawn bags), leached or washed away by rains etc. Adding compost, natural or synthetic fertilizers replenishes these otherwise lost nutrients.

When backyard gardeners talk about fertilizers, they  are mainly referring to  commercial balanced fertilizers that supply N-P-K (Nitrogen, Phosphorus and Potassium), three key nutrients of the macronutrient group. The concentration of these elements is typically cited as percentages of package weight, e.g. 5-10-5, 10-10-10. The amounts of other macronutrients may or may not be mentioned, and micronutrients hardly ever. The N-P-K combo get the spotlight because they are significant in foliar, root and fruit development; but, as hopefully discernible  from the above text, they are not the whole story.

Fresh & Mature Compost.  Source: permaculturenews.org

Commercially produced fertilizers offer the advantage that they tell you the amounts of some or all of the macronutrients you are getting as opposed to what you sprinkle on your garden from your compost pile. Nutrient specific fertilizers also are available to address  specific soil needs, e.g. sodium nitrate or superphosphate, that might have been identified from a soil test.  Commercial fertilizers derived from organic sources, e.g. those containing bone meal, fish meal,   vegetative compost, manure etc.,  will include more nutrients and also be sterile. The best summary about  the types fertilizers available, I have found, is this one from the Weekend Garden Net.   Scroll past the self-test and look are the chart.

Putting your compost on your garden and mulching does a lot to maintain nutrient levels, especially micronutrients; and if you do not observe any plant problems, keep doing it. Just be aware that our backyard compost piles typically show a N-P-K ratio of around 1-.5-1.  Although you might come to think that plants need a 7 course fertilizing banquet, in reality they make do with very little.  For example, if you want to apply a liquid fertilizer to your vegetable garden, Cornell suggests the following ratio of a dry fertilizer (15-15-15) to water: 1 1/4 ounces of dry fertilizer to 2 1/2 gallons of water. This concentration provides a nutrient drink of only a couple of hundred parts per million!   If you were going to use this concoction for seedlings, cut it down to about 1/8 of a teaspoon. [And, REMEMBER: it is always better to use less fertilizer than too much and do not put either dry or liquid fertilizers directly on plants.] To get better guidance about using fertilizers in your garden, see the Monroe County Extension's publication,  Soil Preparation and Fertilizers for the Home Flower and Vegetable Garden.

"If the grass on the other side of the fence appears greener... it must be all the fertilizer they are using." -  Kevin Rodowicz, MD