Powdery and downy mildews are two common pathogens that impact plant and crop health. Powdery mildew is a fungus that results in distinct white spots on plant leaves and stems. It’s common among many plants and crops including legumes, cucurbits, apples, pears, onions, maple trees, and grapes. It requires low humidity with warm temperatures, making greenhouses a great environment to infect. On the other hand, downy mildew is a fungus-like parasite that is actually more closely related to algae. It also results in spots which are distinct in that they are more angular, and often yellow or gray in color. In grapes, downy mildew spots are yellow and oily-looking. In mint and basil, downy mildew spots are darker brown or black. Downy mildew is most common in spring and fall with cool nights and high humidity, warm days.

With both mildews, they survive by stealing nutrients from plants which can stress, weaken and even kill the plant. They can also make plants vulnerable to other pathogens and insect damage. Similarly, they can spread via insects like aphids, wind, rain, runoff, irrigation, and contact with infected plants.

Downy mildew is a zoospore, making it capable of “swimming” through water to infect one plant to the next. Spots on leaves can grow larger and denser until they even impact photosynthesis. This can lead to defoliation, sunburn, soft rots, plant death, and can affect the level of sugars that develop in fruit and vegetables and therefore their flavor. Both also have unique varieties that affect particular crops, for example the variety of powdery mildew that has infected squash cannot infect grapes. It is important to be aware of plant families though, as mildew that has attacked basil can be transferred to mint.

Despite their differences, their management techniques are similar. Because they need high humidity to proliferate, it’s best to avoid over-watering techniques, avoid fertilization during outbreaks, properly space plants particularly in greenhouses, and prune overcrowded areas particularly for trees and crops like grapes. Since downy mildew can overwinter, it is critical to dispose all infected plants. For cucurbits, there are some mildew-resistant crop strains. For grapes, berries will naturally protect themselves after 2-3 weeks of their development. However, certain fungicides like sterol inhibitors and strobilurins don’t completely kill mildew and others like difenoconazole are phytotoxic to grapes.

For grapes in particular, and any other affected plant or crop, there are environmentally friendly fungicides and algaecides that can contain both powdery and downy mildew. BioSafe PerCarb is a biofungicide based in sodium carbonate peroxyhydrate that should be applied every 7-10 days to field crops or within greenhouses. To maximize treatment efficiency, it is recommended to supplement with a foliar treatment such as Oxidate 2.0. These fungicides work together to provide increased stability, and again can be applied from seed to harvest via spray, soil drench, or pre-plant drip with a 0-hour re-entry interval and 0-day pre-harvest interval. They are also both EPA certified, OMRI approved, and biodegradable.

What is the recommended product and application rate to treat water for pathogen suppression in hydroponic & aquaponic systems? As we’ve had a number of customers ask questions on this topic and these applications are not covered in much detail in the specimen labels we’ve decided to cover it in this post.

The recommended product for this application is OxiDate and it can be applied as a direct application to the reservoir at a rate of between 1:1000 and 1:2000 (6.4 fl.oz – 12.8 fl.oz of OxiDate per 100 gallons of water). For any young seedlings a rate of 1:2000 is recommended. For more mature plants a rate of up to 1:1000 can be used. When used in conjunction with a liquid fertilizer the recommended rate is 1:2000.

Since the application of OxiDate at 1:1000 can reduce the pH of water by a few points and root systems vary in their sensitivity to pH changes and phyto-toxicity, we would strongly suggest testing on a small batch of plants using this rate and make sure the plants are ok before applying on a large scale. If any wilting or other phyto-toxicity symptoms are observed, decrease the rate and test again. As a reference, see the attached PDF document that charts PH and ORP values at varying dilution rates.

Once an appropriate rate is determined (1:1000 or 1:2000), apply that rate to the reservoir. If the rate is 1:1000, the solution will have approximately 270 PPM of H202 and if it is 1:2000, it will be around 135 PPM of H202. Monitor the H202 PPM using Peroxide test strips once every 48 hrs and replenish with OxiDate as needed to maintain the appropriate PPM (135 or 270). The dosage calculator helps determine the correct dosage based on the test strip reading.

In aquaponic systems caution should be exercised as we have found that fish cannot sustain sudden pH changes and OxiDate can drop pH depending on the rate and water quality. In this case the manufacturer recommends that a 1:2000 rate is split applied. For example, for 2000 gallons water, at 1:2000, 1 gallon of OxiDate will be applied as 4 applications of 0.25 gallons with a time interval of at least 4-6 hours between each 0.25 gallon application. The pH level should be monitored before and after each application to make sure that pH is not dropping by more than 0.5-1.0 point after every 0.25 gallon application and that at the end of 4-6 hours, the pH of the water is coming back to where it was.

Until the next time – happy growing!

Organic-approved biological fungicides have definite environmental advantages over conventional pesticides, but differ in their mode of action compared to activated peroxygen products.