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.

Some of the most fascinating methods that can be used to protect cultivated lands and managed forests from pests are those that have developed through biological and evolutionary processes. Like all living things, insects have natural enemies from bacteria to toxic plants to other insects. It seems only logical to mobilize these natural enemies as biopesticides for protecting crops, fruit and nut trees, and ornamentals.
Azaguard is a completely natural insecticide that is based in azadirachtin, named from its source the Azadirachta indica or “Neem tree”. This insecticide contains over 100 limonoid compounds. Limonoids, common in citrus trees and the Neem tree, work as an insect growth disruptor deterring egg-laying and as an anti-feedant leading to insect starvation and death. (Limonoids are also responsible for the sour or bitter taste that citrus fruits offer!) The Azaguard insecticide is effective against destructive pests including termites and the particularly invasive southern armyworm. It is also effective against particular insects that are resistant to other commonly used biopesticides such as the Bacillus thuringiensis bacteria. While most effective when applied during the larval stage, it can be applied at any point during larval, pupal, or nymphal stages of insect development. It should be applied at least every 10 days, and is safe for greenhouse or field applications via drop irrigation, soil drench, fogging, or aerial applications. It is immediately effective, has a 0-day pre-harvest interval, and biodegrades in about 4 days after exposure to light or water.
Apart from using limonoids as a weapon, there are even more complex natural processes that won’t only deter insects but actually kill them. BioCeres WP is a mycoinsectide, meaning it is a microbial insecticide that infects insects with a living pathogen that ultimately causes insect death. In this particular case, Beauveria bassiana fungus is spread to insects including whiteflies, aphids, thrips, weevils, and cabbage maggots. Again, this pathogen is a completely natural enemy to the insect, which has been harnessed to control pests in agricultural settings. The BioCeres biological mycoinsecticide works by adhering to the insect’s outer skin, then penetrating their exoskeleton and infecting them with white muscardine disease. The insects then die within a matter of days. The symptoms of this disease are visible on insects, noted by a white foamy coat, although it is also important to note that this pathogenic process can also successfully kill insects before the symptoms are visibly apparent. Unlike other forms of pesticides, it is very unlikely for insects to develop a resistance to mysoinsecticides due to their natural relationship. BioCeres WP can be applied via soil drench or as a foliar spray every 5-7 days. It is effective during all stages of insect development, has a 4-hour re-entry interval and also has a 0-day pre-harvest interval. Both biopesticide solutions are OMRI and EPA approved, with the former also being Kosher. So next time you’re not sure where to turn to claim victory over the insects, look no further than their own natural-born enemies.