In recent years, more greenhouse growers have begun to use pest management products that are organic (OMRI listed) or are considered "minimum risk" by the US EPA. In addition, naturally-derived pest management products available over the counter often have labels bearing words like safe, natural, and non-toxic. Both amateur and experienced growers should be aware that these safety qualifiers typically refer to human health, and organic products can cause phytotoxicity if they overapplied or used improperly. It’s important to keep in mind that the active ingredient is not necessarily the one that causes phytotoxicity - many products contain oils or other “inert” ingredients that may be phytotoxic under the right set of circumstances as well.
A few rules of thumb to remember:
Seedlings and flowers are especially vulnerable. Seedlings are very tender and are more susceptible to spray injury than mature plants. Flowers may also be damaged by sprays even though foliage is not.
Don’t kill your plants with kindness. More is not necessarily better! Be sure to read the entire label and follow the recommendations. When trying a new product/crop combination, spray a few plants first and then wait several days before evaluating them for phytotoxicity.
Be especially careful with copper. Copper sprays can be phytotoxic, especially if the water used is too acidic. Periods of cool, overcast weather also increase the risk of phytotoxicity by increasing the amount of time it takes for foliage to dry after a spray application. Lastly, copper products vary widely in solubility, which affects their persistence on foliage. More information on copper in the August 16, 2018 issue of UMass Extension's Vegetable Notes: http://ag.umass.edu/sites/ag.umass.edu/files/newsletters/august_16_2018_vegetable_notes.pdf
Report by Angela Madeiras