Preharvest Intervals

Late summer is a time when vegetable growers spend much of their time harvesting produce. Many growers, however, also find it is necessary to apply pesticides. All pesticides label state a preharvest interval (PHI) on the label. This is the amount of time, in days, between the time the fruit is sprayed with a pesticides and the time it can be harvested. That is, after a pesticide is applied to a vegetable crop, a specific amount of days must pass before the fruit is harvested. This article will breifly describe how PHIs are determined, give some examples of PHIs and list a couple of questions about PHIs. I have used examples of vegetable crops and fungicides, however, the same concepts apply to apply to all pesticides and all produce.

The reason the US EPA determines PHIs is to ensure that produce that is consumed does not have unsafe pesticide residues. The first step in determining a PHI is to determine a safe pesticide residue for human consumption. This is known as a tolerance. That is, how much of the pesticide, or breakdown product of the pesticide, is a safe level in the fruit or vegetable. Then the EPA conducts studies of pesticides applied to produce at different times before harvest to determine how much time is necessary before the tolerance level is achieved. For example, some vegetables may be harvested the day of pesticide application. Other vegetables may not be picked for several days.

Some examples follow.

  • Fontelis® has a 0 Day PHI on tomato and a 1 Day PHI on cucurbits.
  • Dithane M45® has a 5 Day PHI on tomato and a 3 day PHI for potato.
  • Quadris® has a 0 day PHI for carrots, while Quadris Top® has a 7-Day PHI for the same crop.
  • Bravo Weather Stik® has a 7-Day PHI on garlic and a 14-Day PHI on green bunching onions.

I have had a couple of questions about preharvest intervals that I thought would be useful to share.

Question #1: I have harvested tomatoes with a product that requires a 5-Day PHI. Can I harvest the tomatoes immediately and then keep the tomatoes in storage for  5 days before being sold? This type of situation might occur if the produce were to be kept cool for a number of days before a buyer was found.

Answer #1: The first thing to keep in mind, is that the label is the law. Since the label states do not harvest the tomato for 5 days, the law must be followed. Now, let me explain some details.

When the PHI is determined, it is based on the produce remaining on the plant until harvest because that is the way the assay was conducted. Once the produce is harvested, respiration changes in the fruit and the breakdown of the pesticide may slow. In this case, if the tomato is harvested immediately after application, the respiration of the tomato changes and may modify the breakdown of the pesticide.

Question #2: Can I harvest my watermelon sooner than the 7-Day PHI if I wash it well before sale?

Again, the label is the law. Since the label does not state anything about a quicker harvest if washed, such an exception is not allowed. Here are some details.

A product that is systemic cannot be washed off. Even if a contact product is applied, the PHI assays were not conducted with washing as a variable. It is not known how much the pesticide residue may be reduced if washed. In addition, my produce wash may be different than your wash. Therefore, washing makes no difference to the PHI.

As vegetable growers select fungicides for harvest time, the PHI should be carefully considered in these choices. PHI values are listed in the Midwest Vegetable Production Guide for Commercial Growers.  However, always check the label to make sure you are using the right value.

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