This evening’s schedule calls for applying fungicides to your watermelon. Everything is ready for the application, but now you aren’t sure. The weather report forecasts rain tomorrow. Will the rain tomorrow wash off the fungicide application? Should you wait to apply the fungicide until after it rains?
This question or similar questions are among the most common questions I receive. A similar question would be “Yesterday I applied a fungicide and it rained this morning. Should I re-apply the fungicide?”
Let’s take the question about applying a fungicide before it rains first. The short answer is that it is usually best to apply a fungicide before it rains. Why? Because rain causes leaf surfaces to be wet, a requirement for most foliar diseases, and rain may splash spores from leaf to leaf and from plant to plant. Therefore, if one is able to coat plants with fungicides prior to a rain, the plant is better protected from disease. In short, periods of wet weather are when a plant is most vulnerable to foliar disease. Fungicides, properly applied, may help to prevent or slow the initiation of disease and the spread of disease which commonly occurs during or shortly after a rain.
But let’s look at this question in a bit more detail.
What most growers are worried about is that a rain that falls shortly after a fungicide application will wash off the fungicide. That is, the grower wonders if the fungicide will be rainfast. Rainfast may be defined as: the effectiveness of a fungicide application after a rain event. The best answer to the question of whether a fungicide is rainfast is to read the label. Unfortunately, most labels do not specify whether they are rainfast. See the list below to find some of the labels for which I was able to find rainfast information. Be aware that the information that you find on the product in your possession may differ from what I have written below. And my list is not exhaustive—be sure to check your label.
Table 1: Rainfast information given on the label of selected fungicide labels.
| Fungicide | Rainfast info |
| Approach® | 1-hour |
| Champ DP®, Kocide 3000® | 8-hours* |
| Curzate 60DF®, Cymbol® | 2-hours |
| Endura® | Yes |
| Fontelis®, Vertisan® | 1-hour |
| Monsoon®, Onset®, Toledo® | 2-4 hours |
| Orondis Ultra® | 30-minutes |
| Presidio® | 2-hours |
| Pristine® | Yes |
| Propimax® | Dry before rainfall |
| Quilt xcel® | Dry before rainfall |
| Quilt® | Dry before rainfall |
| Quintec® | 1-hour |
| Rhyme® | 2-hours |
| Tanos® | 1-hour |
| Tilt® | Dry before rainfall |
*This rainfast period was listed only for use on Ginseng.
This list may bring a few questions to mind. For example, what does the word ‘dry’ mean in the table above. I believe it means that the fungicide is rainfast after it has dried on the plant surface. What does ‘yes’ mean? This is less clear. But presumably the manufacturer is willing to stand by the product as rainfast under at least some conditions. A length of time listed indicates that the product is rainfast if applied that period of time before a rain. If a fungicide is not listed, it means I did not find the term rainfast on the label. It is interesting to me that some well-known products with trade names that include rainshield or weather stick have no mention of rainfast on the label. So, it pays to search the label carefully (I have obtained the PDF’s of labels and searched for the term ‘rainfast’ as a way of searching for information).
The amount of rainfastness a fungicide has may be, at least in part, dependent on whether the product is contact or systemic. Contact products such as chlorothalonil (e.g., Bravo®, Equus®, Echo®) or mancozeb (e.g., Dithane®, Manzate®, Penncozeb®) remain on the surface of the plant after application. Such products are more likely to be washed off after a rain. But read on for much more detail. Systemic fungicides move into the plant after application. This characteristic makes these products, in general, more likely to be rainfast. But more detail can be found below.
When I failed to find much information from the label, I looked in the scientific literature. Below, I have listed some of the take home messages from what I found. The format I have used is to list the trade name, the crop and disease investigated and the conclusion of the authors and finally the citation of the publication. My thoughts are given in italics.
Contact products:
Daconil® (chlorothalonil); creeping bent grass and dollar spot; best to apply product 8-24 hours before rain. J. Inguagiato, K. Miele Crop Protection 83 (2016) 48-55
Bravo® 500 (chlorothalonil); potato; rainfall on the day of application resulted in the greatest loss of residue. Later rainfall did not result in as much residue loss. There was some redistribution of fungicide residue from top to bottom of plant with rain. Thus, rain does not necessarily remove fungicide residue from the plant, but may redistribute the fungicide. Applications of chlorothalonil 24 hours before a rain will probably be effective. Bruhn & Fry Phytopath. 72:1306-1312 1982
Dithane Ultra WG® (mancozeb); apple; recommend drying time of at least 4 hours. That is, application of Dithane® 4 hours before a rain will probably be effective. Hunsche et al. crop prot. 2011 30:495-501
Copper hydroxide-copper oxychloride; apple; 0.2 inches of rain washed off 80% of copper. Most copper applications are probably not as rainfast as other contact fungicides. Hunsche et al. crop prot. 2011 30:495-501
Spray adjuvants
The addition of an agriculturally registered surfactant to sprays of Dithane F-45 Rainshield® will improve initial spray deposits, fungicide redistribution and weatherability. Read the label and use adjuvants recommended by manufacturer.
Systemic products
Quadris F® (azoxystrobin); fall fescue and brown patch; effective 15 min after rain; recommend 1-hour rainfast. Butler et al. Crop, Forage and Turfgrass mngt Sept 2019
Quadris F® (azoxystrobin; grapes; 1-day old residue; 2-inches of rain did not reduce effectiveness; Interestingly, the same 2- inches of rain reduced residue, but did not reduce effectiveness. This information is from Annemiek Schilder at Michigan State University in an extension article.
General guidelines
Apply most fungicides on 7-14 day schedule. More frequent applications should be made during or right after rainy weather. MELCAST, a weather-based disease forecasting program for cucurbit growers takes weather into consideration when recommending fungicide application intervals.
For most crops, use a 14-day minimum. After 14 days, the fungicide may have weathered considerably and new growth will warrant a new application.
Apply contact products 24 hours before a rain if possible. As the information above suggests, an application of a protectant even 8 hours before a rain may be effective.
Apply systemics 1-2 hours before a rain. This is a general guideline. Some products labels have specific information.
Check label for more info. Above I have listed several fungicides that have specific rainfast information on the label. Nevertheless, check your label carefully. If a rainfast guideline is given, great. If a specific adjuvant is given, follow that information carefully.
Let’s return to the question I proposed at the beginning of this article. Should I wait until it rains before I apply a fungicide?
In most cases, one should apply a fungicide before it rains. We’ve seen how rain is important is creating leaf wetness and spreading spores from leaf to leaf. Contact products will probably be effective if applied 24 hours before a rain, perhaps even 8 hours before a rain. Systemic products will probably be effective if applied 1-2 hours before a rain. Again, read the label for specifics.
I applied a fungicide yesterday and it rained 1 inch today. Should I reapply the fungicide as soon as possible? In most cases, no. However, it might make sense to schedule the next fungicide application a bit sooner than scheduled. After the rainfastness period (24 hours for contact, 1-2 hours for systemics) older reduces are more likely to need application than more recent applications. Contact products are more likely to need reapplication then systemic products.
In summary, it is best to apply fungicides before a rain if at all possible. If a heavy rain occurs shortly after a fungicide application, it usually makes more sense to shorten the interval before the next fungicide application than to make a second application immediately after the rain.