Hold The Copper

Mango trees are blooming like crazy right now. They are mighty easy trees to grow, at least for those within a few miles of the coast, but they do have one Achilles' heel--- their vulnerability to fungi. Anthracnose and powdery mildew are the big two, but there are also scab, leaf spot, and Verticillium wilt. An extensive collection of photos of these uglies, taken by Scot Nelson of the University of Hawaii at Manoa, can be found at growables.org.

The big two can attack newly emerging panicles, flowers, and young fruit, leading to the disappointing fruit drop that often follows a promising start. What's a dooryard grower to do?

Prevention is always the best place to begin, and good basic practices include selecting anthracnose-resistant varieties and growing in sunny locations where the trees can dry off faster. Also, prune to keep the interior open to airflow, again to minimize fungus-friendly moisture. This carries the added benefit of letting more light reach the lower branches, helping the grower to keep the tree shorter and bushier.

When fungal problems do appear, sulfur and copper are the standard fungicidal options. One application when the panicles are half-size and a second about 10 to 21 days later is a typical recommendation. Elemental sulfur is non-toxic, while copper definitely falls in the toxic category, and can persist and accumulate. Sources are inconsistent concerning the relative effectiveness of these against each of the big two fungi, but the general impression I get is that sulfur is effective against powdery mildew but perhaps not so much against anthracnose. The "safe" recommendation seems to be copper or a combination of the two fungicides.

I reckon there is sulfur in my future. As for copper, never say never, but I'll have to be dragged there kicking and screaming.

For now, I'll make use of the twin luxuries that dooryard growers have and commercial growers don't. First, our fruit can be ugly, as long as it tastes fine. And second, we can fail without going out of business. So this season I'm trying a biological approach using a well-known fungus-fighting microbe, Bacillus amyloliquefaciens. It comes in different strains, and strain D747 is approved for use against papaya anthracnose in Hawaii. At least one strain, DGA14, has worked against mango anthracnose on fruit surfaces in the laboratory.

Last year I obtained some D747 from a large supplier of ag products for the southern region, and I've been spraying it on my mango trees during this flowering season. Of course the great and sometimes insurmountable challenge of horticultural science is to ferret out which of the dozens of variables involved are actually having an effect, so I don't expect anything definitive from my limited sample. But if my larger trees all hold some fruit this year, B. amyloliquefaciens will get a longer look. If not, then there's Bacillus subtilis...