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What's this disease?

Mulberries generally can’t be found on the supermarket shelf, but many southerners enjoy collecting fruits off their mulberry trees to eat fresh and to use in baked goods. While many people think these trees are a nuisance because of the copious amounts of fruit falling to the ground and making a general mess of things, they have many perks too.

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Mulberry infected with the popcorn disease pathogen, Ciboria carunculoides. 

Mulberry trees grow up to 80’ tall making nice wind barriers and shade trees, and they produce a bounty of fruit year after year without much maintenance. They attract a lot of wildlife, quickly turning a backyard into a bird haven. The fruits are sweet and look like slender blackberries.

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Unfortunately, the picture above is a branch from the mulberry tree in my parent’s backyard collected in June this year. Individual carpels of the fruit resembled unpopped popcorn kernels and all the affected fruits did not ripen. The birds and my mom were extremely disappointed!

I diagnosed her tree with popcorn disease, which is caused by the fungus Ciboria carunculoides. Spores of this fungus colonize mulberry flowers and the pathogen grows inside developing fruit causing the carpels to enlarge and harden. Luckily for infected trees, the pathogen only colonizes fruit and the tree itself will remain undamaged.

While popcorn disease isn’t an economic problem on ornamental trees, it can cause large yield losses for commercial mulberry fruit producers (and misfortune for those hoping to collect the sweet berries from nearby trees). So what can be done to save fruit on trees in your backyard? Your options are limited and rely heavily on sanitation practices like removing or burying infected fruit throughout the season. This won’t eliminate infections in following years, but it could reduce the number of infected fruits.

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Infected carpels of a mulberry fruit, which now resemble popcorn kernels. 

Another disease management option is to apply a fungicide called Bordeaux mixture, or another formulated copper fungicide. Bordeaux can be mixed at home by combining 3 ⅓ Tbsp of copper sulfate and 3 Tbsp. of hydrated lime into a gallon of water. The mixture should be made fresh each time and sprayed to completely cover tree foliage, which can be extremely difficult on tall trees with large canopies. In addition, white mulberry and white mulberry tree hybrids are more susceptible to popcorn disease, so planting red or black mulberry varieties could decrease problems with C. carunculoides.

Disease severity varies year to year, so even if you go a few seasons without these delicious berries, all hope for a bountiful year of produce isn’t lost.

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