The Dirty Dozen

In the USA every year the Environmental Working Group (EWG) looks at the results of the US Department of Agriculture’s testing and ranks the “dirtiest” and “cleanest” fruit and vegetables in the supermarket in relation to the amount of pesticide residue they contain, both within, and on the outside.

The “bad” produce includes: apples, celery, peaches, strawberries, spinach, blueberries, and lettuce, while the “good” produce includes: onions, sweet corn, avocado, asparagus, cantaloupe, kiwifruit, cabbage, watermelon, and kumara.

The purpose of this survey appears to be to suggest that only organic produce is safe, and this appears to be a continuation of a misconception. It is necessary in many cases to spray organic crops with pesticides in order to grow a crop of acceptable quality and yield, and not all organic sprays are safe for the consumer. For example, copper is a heavy metal and should always be used sparingly to avoid residues on the crop and also in the soil.

I do not, however, have too many problems with the inclusion of strawberries on the bad list, because in any country (such as New Zealand) when it can rain during the harvest season, it is absolutely necessary to spray with pesticides regularly in order to protect against fungal problems such a botrytis fruit rot.

However, as strawberries (and other berry fruit such as raspberries and blackberries) must be harvested sequentially as they ripen, there is no way that spray residues are not on the fruit, and washing this type of fruit is not an option.

Of course, it can be argued that using pesticides with a short withholding period will cover this, but no pesticide is absolutely safe for everyone, and it would be preferable to use pesticides (whether approved for organic or conventional production) as little as possible, particularly during the fruit harvesting period. Last year, in a very wet summer some outdoor strawberry growers were found to have excessive pesticide residues on their fruit.

Fortunately, the solution is relatively straightforward, as we discovered in our greenhouse strawberry studies that by putting a roof over the crop (a rain shelter) immediately changes the situation. This can be further improved by growing the crop above the ground hydroponically (using the so called a tabletop system). Botrytis cinerea (the fungus, which causes grey mould on strawberry fruit) does not initially infect fruit, as it predominantly gains entry via the dead flower petals, so that as a further precaution a single spray with a fungicide at flowering would act as an additional insurance, although we have never done this. I should add that in our greenhouse (tabletop) strawberries, we have harvested only a handful of rotten fruit from several hundreds of kilos of ripe fruit, without applying any fungicides or pesticides.

We have used the regular applications of biologicals (supplied by Bioforce) to control two-spotted mites, cyclamen mites and thrips. Phytoseiulus persimilis for two-spotted mites, Neoseiulus cucumeris for thrips and cyclamen mites. Although what we are doing cannot be classified as organic, as we are growing hydroponically using conventional nutrients, it certainly could be described as pesticide free. This is (apparently) the main reason why people who buy organic choose organic rather than conventionally produced fruit and vegetables.

About the author
Dr Mike Nichols is a retired lecturer from Massey University and a regular contributor to Practical Hydroponics & Greenhouses magazine. He has travelled around the world consulting on horticulture and is one of only 25 honorary members of the International Society for Horticultural Science (ISHS).
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PH&G August 2017 / Issue 182