Phytotoxicity causes and how to minimize the risk

Phytotoxicity occurs when a substance causes a toxic effect on plants. Phytotoxicity can occur when chemicals are applied, such as in foliar spraying and irrigation systems, to protect plants from diseases, pests, and other hazards. Additionally, phytotoxicity can occur following applications of fertilizers or growth regulators. There are no control measures; therefore, the focus should be on prevention.

Symptoms of phytotoxicity

  • Burn and necrosis.
  • Chlorosis.
  • Distortion, such as cupping or twisting (might be confused with a viral disease).
  • Stunting and abnormal growth (root damage or poor germination).
Phytotoxicity symptoms on basil leaves
Phytotoxicity symptoms on basil leaves
Phytotoxicity symptoms on cucumber leaf
Phytotoxicity symptoms on cucumber leaf
Phytotoxicity burn and distortion symptoms on basil leaves
Phytotoxicity burn and distortion symptoms on basil leaves

How to differentiate phytotoxicity from other damages

Symptoms of phytotoxicity can be confused with damages caused by plant pathogenic organisms or genetic disorders. Look for the following patterns to identify damage due to chemicals:

  • Leaf damage patterns show sharp edges with no discoloration gradient.
  • Plants very close to the sprayer show more damage than plants further away from the sprayer.
  • Symptoms show up fast in a wide area (1-7 days after application), and there is no further spread after the initial show-up.
  • New growth will appear healthy.

If there is a doubt, upload images of the affected plant to the Agrio app, and our artificial intelligence will help you with the identification.

Phytotoxicity causes

Environmental conditions

We discussed in much detail the environmental conditions that increase the risk of phytotoxicity in another blog post. In high temperatures (or water-stressed plants), low and high humidity are among several weather factors pesticides should be avoided. We refer the reader to the blog post to read how Agrio can help you time pesticide applications to minimize injury risk.

Agrio can help you time pesticide applications to minimize phytotoxicity risk.
Agrio can help you time pesticide applications to minimize phytotoxicity risk.

Growth stage of plants

Seedlings, flowering and fast-growing are more sensitive. Time the pesticide applications accordingly. This is especially important when herbicides are applied.

Chemicals applied improperly

If this is the first time that you are using a new pesticide, you should refer to the label of the product to familiarize yourself with the instructions. Check the label of the chemical and make sure the crop being treated is listed. Certain plants are sensitive to certain chemicals, so be careful what you use.

Mixing incompatible chemicals should also be avoided, and the product label provides information about that. Other common causes that we encounter are the application of sulfur and oils in close time proximity (less than a one-month window) and high dosage applications. Sulfur can stay on plants for a long time, and oils interact with it, forming phytotoxic compounds.

Note that high dosage can be in one application or an accumulation of several sequential applications. Apply the chemical thoroughly and evenly to avoid high dosages in some patches.

When testing a new pesticide for the first time, it is advised to apply it on a few plants every few days to get a sense of the amount of pesticide that can cause damage to the plants.

Pay attention to the formulation. Dust and wettable powders are less harmful options than emulsifiable concentrates. Emulsifiers can react with plant tissue and cause unwanted damage. Adjuvants such as spreaders, stickers, and wetting agents can increase the risk of an injury as well.

We recommend devoting separate sprayers to herbicides. Rinse the spraying tanks between applications and calibrate your spraying equipment once a year.

Drifts from the target crop to a sensitive crop

If you practice intercropping or growing different crops in proximity, you should pay attention to drift and the non-target crop sensitivity to the applied pesticide. Examine the field drainage routes and avoid applying pesticides when there is a risk of run-off.

Avoid pesticide spraying on windy days
Avoid pesticide spraying on windy days

Residues accumulate in the soil as a cause of phytotoxicity

Repeated applications can result in the accumulation of active ingredients to a toxic level. You should refer to the product label and follow the annual rate restrictions.

If you practice crop rotation (as you should!), you need to plan the next planted crop according to the risk of residues of pesticides and herbicides in the soil. Field crops that are produced after the harvest of a pesticide-treated primary crop might be affected by soil residues. Refer to the label and check the annual application rate limit to see if the crop to be planted next can be in danger.


Plants are subject to a variety of injuries, and many of these injuries are caused by human beings. The most common human-caused injury is the use of pesticides and herbicides, which can be detrimental to plants in many ways. By following the product label instructions and implementing good practices in the farm and garden, growers can avoid such damage.