Rust-ication

Carolina Buckthorn rust - Lisa Berger

Rusticate - go to, live in, or spend time in the country.

When we first acquired our tree farm at Bull Mills, our neighbor Harry would call out "Rusticating again?" as we packed up for the weekend.  The implication was heading out for a life of leisure although over time it morphed into more work as most things do. 

"Rusticating again" come to mind in an email from Lisa Berger with the picture above of a Carolina buckthorn (Rhamnus caroliniana) leaf showing a plant rust.  Spring and early summer is the time when leaves show the signs of rusts, plant diseases caused by fungi of the order Pucciniales.  Over 180 genera  and 7,000 species of these fungal parasites exist, damaging leaves but rarely killing their victims.  Our concerns are more cosmetic while farmers suffer far more in crop losses.

Life cycle - Forests CC

Rusts fungi have a complicated life cycle, producing up to five different spore types depending on the species and circumstances.  Most of the literature focuses on agricultural species because of their economic importance.  A typical life cycle is shown above for stem rust caused by  Puccinia graminis whose host shrub is barberry Berberis vulgaris (barberry).

Lisa's rust agent is likely Puccinia coronata, first found on barley in 1922.  It also attacks oat and rye grass and causes oat and barley crown rust.  Its alternate host plant is common buckthorn, Rhamnus cathartica, the same genus as Lisa's Carolina buckthorn, Rhamnus caroliniana. 

Buckeye rust - underside of leaf

Top side

I first got interested in rusts with the yellow bumps on buckeye leaves that I saw all along the Ozark Mail Trace road last spring.  These yellow spots are the aecial stage of the rust Puccinia andropogonis. 

This rust uses grass as an alternate host, probably the big four species of warm season grasses we had planted nearby.  The common alternative host plant is big bluestem (Androposen gerardii).  The effect on our buckeyes is cosmetic only, in our case adding a bit of color and a lot more interest to the variety of life along Bull Creek.