More On Mango Fungicides

A couple of weeks ago we examined some of the options for fighting mango fungi, especially the big two, anthracnose and powdery mildew. It's an important topic for us mango growers, and this is the critical time of year for forming and holding fruit, so let's give it another look.

The Washington State University investigated a number of organic fungicides registered for home use and compiled an informative table about their effectiveness and side effects. Most are available from makers of garden products such as Bayer Natria, Earth Tone, Garden Safe, and Worryfree. Bearing in mind that the focus was not on mango production but rather on home gardening, here are some notable items:

---Bacillus subtilis (sold as Serenade) - Low to moderate efficiency of up to 50% disease reduction. Best results are when used to treat powdery mildew at the first signs of disease.

---Canola oil - Very high (near 100%) effectiveness. Acts directly and locally. May cause damage in high concentrations.

---Neem oil - Moderate to high efficiency, from 60-90%. Not long lasting, not known to damage plant tissues. One of its components is an insecticide that should be used with caution if bees are present. (Maybe we'll find out more from our next MRFC speaker.)

---Potassium bicarbonate (sold as Bi-Carb Old Fashioned Fungicide) - Moderate to high efficiency, highest on smooth surfaces. Only effective on contact, and may damage plant tissues.

A recent thread at the Tropical Fruit Forum discussed copper-based fungicides. A veteran grower in Broward County, self-described as “bio-sensible”, makes the case for copper. On a half acre of 30 mostly large mango trees, he uses a mix of copper octanoate and elemental sulfur, which he says contains about a quarter as much copper as regular liquid copper fungicide. He says that he can spray all of his trees using just two gallons of mix, making the total amount of copper “miniscule”.

In more detail, Broward calculates that the ½ cup of copper octanoate used for one spraying contains only about 1/15 of an ounce of copper, compared to a quarter-ounce in a bag of palm fertilizer. Alternatively, one gallon of concentrate--- enough for an entire year in his grove--- contains about 0.15 pounds of copper, below the recommended application rate as a soil nutrient. He also notes that according to the label, copper soap is allowed on organic vegetables up to the day of harvest.

To accomplish this efficiency in his spraying, he uses a thousand-dollar mister (pump included). I suspect that my $15 hand pump sprayer would need a lot more concentrate to do the job.

A very, um, outspoken mango professional (TFF readers will know of whom I speak), who rarely leaves Broward's comments uncontradicted, says

“Many years ago people used copper to treat powdery mildew. Like humans and taking penicillin, copper ceased to be effective for the treatment of powdery mildew. Sulfur is now the `drug of choice' for treating powdery mildew.”

And what does the Washington State Extension say?

---Copper octanoate - Potentially high efficiency of 74% in greenhouse trials. “Material may persist in soil, but potential effects are not documented.”

---Sulfur - Low efficiency by itself, but seems to boost the effectiveness of other fungicides. Some may be absorbed as a plant nutrient, strengthening the plant's natural resistance.

And now that I've made everything perfectly clear, it's your call!