Mildew on mango: chemical and organic controls all have impacts

My Keitt mango tree is in full flower, and infested with the same powdery mildew that last year killed off every flower or fruit.

The fungus, Oidium mangiferae, infests the flowering portions and young fruit, killing them and turning them black. 

Instead of their normal vibrant yellows and pinks, the flowering stalks of infested mango turn brown and gray. The fungal disease is a main culprit in mango yield in the Islands.

In researching solutions, I entered the bizarre world of plant disease control, in which few options are without unwanted impacts, and the safest alternatives are illegal.  

 If you don’t care to read through this, the takeaway is that every recommended anti-mildew product has environmental and/or health impacts, whether it is synthetic or natural, chemical or organic. But while they all have impacts, the impacts can be quite different.

Even the option of doing nothing is hazardous, since fungal spores can cause breathing problems.

There are all kinds of ways to approach fungal disease issues on plants. Here are the main ones:

Biological control (living creatures that attack the living creatures causing the disease); cultural control (keeping the area clean so things like rotting leaves don’t harbor disease agents); chemical control (using an array of natural and manufactured products to kill off disease agents); natural resistance (selecting species that fight off the disease—which is, of course, no longer an option when you’ve got a mature fruiting tree); integrated pest management (which can be blending any of the above.)

The University of Hawaii Cooperative Extension Service has a leaflet on powdery mildew on mango, which it says can reduce fruiting in mango by 90 percent. It’s a bigger problem in areas that have rain during the mango flowering season.

Each possible treatment has its downsides as well as its benefits. Some only work well in conjunction with others. Some barely work at all.

Says the leaflet: “The fungicides registered for control of mango powdery mildew in Hawai‘i fall into several groups based upon their active ingredients: clarified hydrophobic neem oil; mono- and dipotassium salts of phosphorous acid; carbonic acid, monopotassium salt; kerosene (petroleum) hydrodesulfurized; aliphatic petroleum solvent; sulfur; mancozeb; and myclobutanil.”

Two of the safest anti-fungal approaches ironically are illegal for use in Hawaii: baking soda and milk.

“Baking soda (sodium bicarbonate) mixed with water is an old home-remedy spray for powdery mildew. However, because baking soda is not labeled as a fungicide, it may not legally be used for disease control, according to Hawai‘i Department of Agriculture regulations. Some growers report that foliar sprays of milk can be effective against powdery mildew, but the same use restriction may apply,” said the UH leaflet.

Neem oil might be most folks’ first choice, although it is moderately toxic to bees and can irritate the eyes.  And if there’s an aquatic environment nearby? “The compound was more or less toxic to all the tested species,” says a study of neem toxicity to aquatic species including insects, crustaceans, amphibians and fish.

Hydrodesulfurized kerosene and aliphatic petroleum solvent are both fossil fuel products with various risks, including skin irritation, and they can be hazardous to aquatic animals.

Some recommend a natural product called potassium bicarbonate for powdery mildew. It can be found in commercial products like GreenCure. According to one product, Kaligreen, the products work by disrupting the potassium or sodium ion balance in the fungus and causing cell walls to collapse. 

There’s some evidence of eye and skin irritation in lab animals, but these may be the safest of the alternatives: 

“EPA has concluded that aggregate exposure to sodium bicarbonate or potassium bicarbonate over a lifetime will not pose appreciable risks to human health. EPA concludes that there is a reasonable certainty that no harm will result from aggregate exposure to sodium bicarbonate or potassium bicarbonate residues.” 

(Despite that finding, of course, sodium bicarbonate or baking soda remains off-limits for powdery mildew on mango in Hawai`i.)

Researchers in Egypt experimented with integrated pest control in seeking out environmentally friendly responses to the powdery mildew on mango, and found that none of the known natural solutions worked really well alone. So they tried mixing several different substances together. They combined three biological control agents (Verticillium lecanii, Bacillus subtilis and Tilletiopsis minor), plus a soluble potassium salt (monopotassium phosphate), plus a clay product (kaolin), and ascorbic acid.

They worked with two mango varieties, and despite the significant complexity of the treatment, combining all of them worked better than any of them alone: “Mixtures of all four natural compounds were more effective in significantly reducing powdery mildew severity and conidia counts on blossom clusters and fruit set and increasing fruit set and yields on trees of both cultivars than mixtures of two or three or single applications.”

Oh, and those natural biological agents they tested? They’re pretty safe, but in certain circumstances can also be problematic.

Verticillium lecanii is a fungus that is sometimes used for biocontrol, but it can kill some insects.

Bacillus subtilis is a bacterium used as a fungicide, and incidentally is used to make the antibiotic bacitracin. It has been associated with liver damage in humans.

Tilletiopsis minor is a fungus that has been known to cause fungal infection in humans. 

Those are “organic" solutions. The chemical solutions also come with warnings. 

The systemic fungicide myclobutanil is listed as only slightly toxic  but there are warnings about eye and skin contact and inhalation.

The non-systemic fungicide mancozeb is listed as not acutely toxic, but may enter groundwater and may be an endochrine disruptor. It is toxic to fish and some other creatures. 

The takeaway, as mentioned at the start of this article, is that every recommended anti-mildew product has impacts, whether synthetic or natural, chemical or organic. 

Nothing is without impacts. Indeed, even doing nothing at all has impacts. 

© Jan TenBruggencate 2015