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How To Diagnose Abiotic Plant Disorders

How To Diagnose Abiotic Plant Disorders

By High Mowing Seeds
Photographs courtesy of the University of Maryland

There are a myriad of ways a plant can show signs of abnormalities. Sometimes these abnormalities are due to the worst causes – namely harmful pests or diseases. But other times, the symptoms are emblematic of an underlying cause, either cultural or environmental, and can often be reversed as long as diagnosis is correct and treatment is swift.

When a plant is suffering from cultural or environmental stresses, these disorders are called abiotic, meaning they are caused by physiological (non-living) elements rather than biological (living, or biotic) activity. Some causes of abiotic disorders include weather, soils, chemicals, mechanical injuries, cultural practices or genetic traits. Abiotic plant problems can sometimes look very much like biotic plant problems, which is why learning some of the common environmental stresses can be a huge help to a gardener attempting to diagnose an abnormality on a plant. The more familiar you are with abiotic diagnosis, the less likely you will be to resort to last-ditch efforts like sprays and fertilizers that may do more harm than good in your biologically active garden.

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This table was adapted from the University of California’s Department of Agriculture and Natural Resources, and can help you make some initial determinations about whether a problem is abiotic or biotic. Overall, one of the quickest and easiest ways to determine whether you’re looking at an abiotic or biotic problem is by examining the newest tissues of the plant. If the new tissues look healthy, and all the old tissue of the same age looks bad, the problem is (or more accurately, was) most likely abiotic.

Air Pollution/Ozone

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Pollution damage on bean plant foliage.

Symptoms: Tip burn; withered leaves; white or tan flecking; bronzing; bleached leaves; purple leaves.

Cause: Air pollution damage on plants is caused by a variety of pollutants in the air. The severity of air pollution damage can vary based on plant species, time of day, heat, wind, sun, and soil type.

Diagnosis: Stay alert to air quality notifications in your area to determine if ozone might be a factor of your plant’s stress. The symptoms listed above often most severely affect older leaves and leaves exposed to direct sunlight and will subside as air quality improves. Damage usually occurs when ozone concentrations are highest and during the hottest part of the day.

Blossom End Rot

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Blossom end rot on developing paste tomatoes.

Symptoms: Dark blemishes on the bottom of fruit that enlarge until the whole bottom of the fruit is dark, shrunken and leathery.

Cause: Blossom end rot is a very common disorder affecting tomato, pepper, eggplant, pumpkin, squash and watermelon plants. It is caused by calcium deficiencies in growing fruit. Because calcium is taken up by plant roots as a dissolved nutrient, inconsistent soil moisture can be a contributing factor to blossom end rot even if calcium is present in the soil. Imbalanced pH levels and excessive nitrogen fertilization can also contribute to blossom end rot.

Diagnosis: Blossom end rot is distinctive, recognizable by the obvious blackened ends of maturing fruit. It is commonly seen in drought conditions, or when watering has been uneven or shallow. Affected fruit will not grow out of the condition, but when moisture levels and calcium uptake are corrected, any newly developed fruit will mature normally.

Fruit Or Root Cracking

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Cracking on tomatoes.

Symptoms: Cracks or splits in mature fruits, roots, or the harvestable portion of vegetative plants (i.e. cabbage heads). Cracks can sometimes heal over and become coarse, not significantly affecting fruit quality other than aesthetically, or they can remain open and moist, becoming a high risk spot for rotting.

Cause: Excessive levels of nitrogen (over fertilizing) or uneven watering (usually caused by excessive rainfall or even heavy dews) are the major causes of cracking and splitting. Some cultivars, like jalapeño peppers, are actually genetically predisposed to develop superficial cracks on the outside of their fruit, which does not affect quality.

Diagnosis: Note if cracking occurs just after fertilization or compost applications and adjust your fertility plan accordingly. Overly rainy or damp weather conditions can also cause cracking; if a large amount of rainfall is expected, consider harvesting fruits prior to full ripeness to avoid cracking.

Fertilizer/Pesticide Burn

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Fertilizer burn on bush beans.

Symptoms: Leaf burn; distortion; yellowed leaves; bleached leaves.

Cause: Misuse or misapplication of fertilizers or pesticides can cause damage to plants. Misapplications include leaving fertilizer on plant leaves; using pesticides in combination with other non-compatible amendments; using pesticides or fertilizers when the weather is too hot, humid, or overcast. Pesticide burn can also be caused by unintended pesticide drift.

Diagnosis: Fertilizer or pesticide burn (yes, even non-synthetic pesticides like insecticidal soaps and horticultural oils) is usually easy to diagnose if you are aware of the timing of your last application and when the symptoms appeared. If applying dried fertilizers, be careful not to leave any residual fertilizer on the plant leaves. Spray application of fertilizers or pesticides should always be used according to the product label to avoid mismanagement.

Herbicide Damage

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Distorted tomato foliage from herbicide damage. 

Symptoms: Depending on the herbicide and its selective action, symptoms can include mottled leaves, distortion of veins, spots or blotches on leaves, and root stunting or swelling.

Cause: Herbicide damage can be easy to diagnose since there are no organically approved herbicides, so any damage that happens to plants in an organically managed system would be due to herbicide drift. It is important to be aware of the land management techniques of the surrounding area when planting a farm or garden in order to assess how the practices of others might affect your plants.

Diagnosis: Many herbicides are selective in their targeted actions, meaning they may only affect plants of a certain species or subspecies. When assessing damaged plants, it is important to note if the damage seems to be selective (only affecting certain plant species) or broad (affecting numerous plant species) to determine if a likely cause could be herbicide drift.

Moisture Imbalance

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Wilted squash plant.

Symptoms: Long periods of wilting; repeated wilting; stunted growth; reduced flowering; lack of vigor; yellowing leaves; shedding leaves.

Cause: Either drought conditions or waterlogged soils. Drought causes heat stress to plants and can result in flower or fruit abortion. Waterlogged soils prevent plant roots from accessing necessary oxygen and will quickly cause wilting, leading to disease issues.

Diagnosis: Wilting can be a cause of either drought or waterlogged soils, so diagnosis is dependent on current weather conditions. Sometimes wilting can happen for short periods only and will not harm plants. However, extended periods of drought or waterlogged soils indicate that the plant is not receiving an appropriate soil moisture balance.

Nutrient Imbalance

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Phosphorus deficiency in a tomato plant.

Symptoms: Plants with nutrient deficiencies may show signs like stunted growth, yellowing leaves, or shedding of leaves. Plants with nutrient excesses (most commonly excess nitrogen) will turn an overly deep shade of green or produce excessive amounts of foliage without producing blossoms.

Cause: Extreme levels of available nutrients, either in excess or deprivation.

Diagnosis: Symptoms differ depending on cause, though you may also be able to determine the cause based on your soil fertility plan and/or known applications of soil amendments. Some common deficiencies and their symptoms include phosphorus (purpling leaves); calcium (tip burn/browning/blossom end rot); and nitrogen (yellowing leaves). Nitrogen is the most commonly over applied nutrient; check the NPK ratio on your fertilizers before applying, and always use soil amendments based on soil test results.

Poor Pollination 

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Poorly pollinated cucumbers.

Symptoms: Malformation of fruits, especially in the cucurbit family (cucumbers, squash, watermelon).

Cause: Uneven or poor pollination. Cucurbits require cross-pollination to produce fruits, which means insects must move pollen from a male flower to a female flower. If not all of the female flower’s ovules are pollinated, the resulting fruit can be malformed, usually smaller on the end where the ovule was not pollinated. Corn can also form unevenly if pollination is not sufficient, though corn is wind-pollinated as opposed to insect-pollinated.

Diagnosis: Fruit with uneven forms are an indication of poor pollination, and usually appear every once in a while in any crop planting, even a healthy one. If your plants are healthy but continue to produce insufficiently pollinated fruit, consider attracting more pollinators to your farm or garden by planting attractants or establishing your own population.

Sunscald/Sunburn

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Sunscald on tomatoes.

Symptoms: Fruit shoulders or walls appear bleached, yellowed and/or blistered. Especially susceptible crops include tomatoes, peppers, watermelon, muskmelon, summer squash and zucchini.

Cause: Too much exposure to direct sunlight, usually caused by insufficient foliage shading.

Diagnosis: Sunscald is usually a common symptom that follows disease, since it is directly related to the coverage provided by foliage. Diseased foliage that dies back or becomes stunted cannot provide enough shade from the heat of the sun to protect delicate fruits. Fruit will not grow out of sunscald, so affected fruits should be removed and a shade cloth or other partial shade element should be installed to protect developing fruit. Fruit affected by sunscald will eventually rot, so it should be removed from plants as soon as possible.


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