Spring Humidity Can Trigger Rust in Almond Trees

We’re all familiar with rust. You see it on old tin cans, aging pipes and junkyard cars. 

   

Merriam-Webster Dictionary defines rust as reddish substance that forms on iron or some other metal usually when it comes in contact with moisture or air.”

In the plant world, it’s another story: “A disease that causes plants to develop reddish-brown spots.” That can be bad news for farmers.

As scientists officially bid farewell to the fizzled El Niño weather pattern last week, the spring rains coupled with hot weather has transformed almond orchards into canopy-covered steam rooms. That creates ideal conditions for rust to develop up in almonds.

“It has been usually humid this year compared to past years,” says Field Scout Jenna Mayfield. “Rust has been a problem around the Valley.”

Rust spots are showing up on leaves. - UC IPM photo

During her rounds through almond orchards, Jenna has spotted evidence of rust in trees. She’s finding small, yellow spots under leaves and on the fruit. The rust-colored spores are spread through the air. If left unchecked, “it can defoliate the tree.”

Rust is most often found in orchards near rivers or streams or other areas with high humidity during the late spring or summer, according to UC IPM. The rust fungus survives from one season to the next through infected leaves.

Almond leaves on the ground because of rust problems.

 To manage this disease, UC IPM recommends for orchards with a history of rust:  “Apply sulfur or maneb five weeks after petal fall and follow four to five weeks later in late spring and summer with a Quinone outside inhibitor fungicide (FRAC Group number 11) to control leaf infections. Two or three applications may be needed in orchards that have had severe rust problems. To be effective, fungicide must be applied before rust symptoms are visible.”

“When zinc sulfate (20-40 lb/acre) is applied in late October to early November to hasten leaf fall, rust inoculum is prevented from increasing. Otherwise, the inoculum may build up, overwinter on the trees, and infect leaves the following spring,” UC IPM says.
Meanwhile, Jenna says growers are out of the woods at this moment for stink bugs and leaffooted plant bugs. They become problematic again at hull split. “We’re waiting for the mites to come up later this month,” she says.

Field Scout Carlos Silva says alfalfa growers have wrapped up their third cutting of the season.  He continues to discover “a ton of lygus”  in alfalfa. Again, now’s the time for growers to leave strips of uncut alfalfa to provide a habitat for lygus bugs and keep them from migrating to nearby cotton fields.

The first post-planting irrigation has started in cotton fields.

Cotton plants continue to develop their first squares with some plants showing four to five squares. Carlos is still finding lygus bugs in some fields. Growers are irrigating the crop for the first time since planting. Overall, things are looking good so far for cotton.