Stigmatomycosis facts for kids

Stigmatomycosis is a plant disease. It's caused by tiny fungi. This disease affects many important crops. These include cotton, soybean, pecan, pomegranate, citrus, and pistachio. It's been found in places like Greece, Iran, and Russia. In California, it's a big problem for pistachio farms. Especially where there are many stink bugs and other insects. A study in 1989 showed it was common in California pistachios. It was found in most samples from June to September.

What Stigmatomycosis Looks Like

Stigmatomycosis makes the inside part of the nut, called the kernel, look and smell bad. It can be wet, stinky, and slimy.

Here are some ways a sick kernel might look:

• It might be small and dark green. It could be only partly grown. A brown stem-like part, called a funiculus, might be attached.
• It could be fully grown but still dark green and smell bad.
• It might be full-sized but look strange. It could be white or light yellow. It might feel like jelly and have a bumpy shape.

Sometimes, kernels can also have "kernel necrosis". This is a dry, brown, spongy area. It's caused by larger bugs. A kernel can even have both problems at the same time.

The Fungi That Cause Stigmatomycosis

Stigmatomycosis is caused by certain types of mold-like fungi. These fungi are called Ashbya gossypii, Eremothecium coryli, and Aureobasidium pullulans. All of these fungi belong to a group called ascomycetes.

How the Disease Spreads: The Life Cycle

The fungi that cause stigmatomycosis are linked to insects. These insects are often stink bugs. They belong to families like Pentatomidae and Coreidae. These bugs are common pests in pistachio farms. Up to ten different kinds of stink bugs have been found in California pistachio orchards.

Some common stink bugs, like Thyanta pallidovirens, Chlorochroa uhleri, and Chlorochroa ligata, can spread the disease. A type of leaffooted bug, Leptoglossus clypealis, can also spread it. They carry the fungus E. coryli. When these bugs feed, they pass the fungus to the pistachio kernels. This causes the typical symptoms of stigmatomycosis.

The disease usually starts to show up in late June. This is after the pistachio shells become hard. But it becomes much more common from July through September. This is when the kernels are growing inside the shells. Smaller bugs, like Lygus and Calocoris, might carry the fungi. However, they usually can't spread the disease. This is because they can't poke through the hard fruit skin after May.

Stigmatomycosis is often worse in orchards that use sprinklers for watering. It's less common in orchards that use drip, microjets, or flood irrigation. This might be because the fungi need more humidity to grow. Or perhaps there are more stink bugs in sprinkler-watered areas. It could also mean there are more fungus spores ready to spread.

How to Control Stigmatomycosis

Using fungicides (chemicals that kill fungi) does not help control stigmatomycosis. However, using insecticides (chemicals that kill insects) can help. Insecticides reduce the number of bugs that spread the disease. This then lowers how often stigmatomycosis happens.