Hello,

I checked my latest references dated 2018 and 2019, and found no new treatments. Work is ongoing in the crop industry to develop resistant oat crops and some other grasses such as fescue, but nothing for buckthorn.

Unfortunately, the rust may be overwintering inside the buckthorn, so it could be an ongoing battle. Rotate different fungicides, starting early spring, labeled for rust so that the rust species doesn’t develop resistance. Timing of your sprays is critical( start early as leaves start sprouting)  and repeated spraying ( read the label) is important. On plants of your size this may not be practical. Many other plants harbor this rust and so your environment may have many fungal sources near by.

Bordeaux mixture is not listed to control rust. Only use products that specifically state they help control rust. Many of these are sulfur or copper based. The ornamental lists aren’t necessarily going to address this particular rust. Here is a list-

http://ipm.ucanr.edu/PMG/r280101211.html

Here are excerpts from a couple recent references, should you like to read them:

“Buckthorn (Rhamnus cathartica) is a tall shrub/small tree that is sometimes becomes large enough to be confused with crabapples (except buckthorn does not have a showy flower and the fruit is a small purplish-black berry rather than a crabapple). Buckthorn is considered a weed, not a valuable ornamental. However, to answer the questions, the disease, crown rust, alternates between buckthorn and cereal crops and grasses. While buckthorn is the primary woody host, the disease is also present on Elaeagnus species such as silverberry and Shepherdia species such as buffaloberry. There is no control for the disease.

From- https://extension.sdstate.edu/tree-rust-diseases

From the “. Diseases incited by Fungi: Rusts” section in the next link——In milder climates, the fungus may over­ winter as mycelium in the host or as uredial pustules that resume sporulation in the spring.

HOST RANGE

The host range includes species of Agropyron, Agrostis, Arrhenatherum, Bromus, Dactylis, Elymus, Festuca, Lolium, Phalaris, Phleum, and Poa (Cummins, 1971).

CONTROL

Resistant cultivars offer an appropriate means to control crown rust in economically important Gramineae. Braverman (1967, 1986) recorded many cultivars, comprising several grass genera and species, developed specifically for crown rust resistance.

From- https://extension.psu.edu/diseases-of-forage-grasses-in-humid-temperate-zones

I am including references for alternative plants to buckthorn, should you decide to replace them.

http://www.uvm.edu/pss/ppp/articles/buckthorn.html

Buckthorn Alternates

* upright shrubs used as unpruned visual screens (Juniperus, Thuja, Viburnum x rhytidophylloides, etc.)

* deciduous shrubs with very fine-textured foliage (shrub members of the Willows, including Salix purpurea)

* evergreen companion shrubs with fine-textured foliage (dwarf shrub members of the following genera: Abies, Chamaecyparis, Juniperus, Picea, Pinus, Taxus, Tsuga, etc.)

I’m sorry there isn’t better news for you. It sounds like you are doing other things to reduce the spores, like cutting out the infected oarts and cleaning up leaves.  Thanks for using our service.