White Pine Blister Rust and Ribes
White pine blister rust (wpbr) is a complicated disease with a great story. The rust is a fungus, cronartium ribicola, that requires two host plants to complete it’s life cycle. The disease is able to kill all 5 needled pines. When it was discovered that ribes (gooseberries and currants) served as the alternate host, the lumber industry waged war on ribes populations.
There are more than 70 species of the ribes genus native to North America. They are small shrubs that often thrive in the shade of forests. The berries have fed woodland songbirds and rodents for millenium. The color of currants and gooseberries can range from translucent white to green to red to purple to black. The flavor can vary just as much from, terrible to delicious. Many cultivars of both native and Eurasian species abound in gardens and nurseries across the temperate world.
White pine blister rust does not effect ribes too much, but it often kills young white pines and injures older trees. In some stands mortality of young white pines can be near 100%, though more and more resistant pines are showing up and being bred.
Originally from Asia, wpbr was introduced into Europe and then brought over to the U.S. accidentally. In the eastern U.S. towering white pines dominated the landscape. The lust for their valuable timber led to their rapid destruction. The trees were used to build first the british and then the American navy ship masts. White pines were so valuable in colonial times that it was illegal for colonists to cut them down. But down they came in great numbers and by the late 1800s it was realized that our forests might not last forever. Replanting programs began, but American nurseries were unable to keep up with the demand for white pine seedlings. We looked to Europe, already well practiced in replanting forests, to supply us with white pine seedlings. Many thousands of seedlings were shipped from nurseries in Germany, and along with the trees came white pine blister rust.
It was first discovered in Geneva, NY in 1909. Very soon afterwards ribes eradication efforts began. WPBR has two different types of spores. One type moves from white pines to ribes plants, and is capable of traveling 100 miles or more. The other type of spore travels from ribes to pines and is only capable of traveling a few hundred yards- and here lies the weakness in the disease. By removing all the cultivated and wild ribes from the immediate area of a valuable white pine stand, the pines could be protected.
Growing ribes became illegal under federal law (this ban was lifted in 1966 with individual states taking over from there). Work crews hired by the government, mostly through the Civilian Conservation Corps (CCC) systematically swept through and around valuable white pine stands uprooting gooseberries and currants. At one point in the 1930s there were over 11,000 men ripping ribes plants out of the ground. It is hard to imagine how many plants were destroyed, and what effect this had on local ecosystems.
Wild ribes are far more common than most people realize. They are inconspicuous plants that thrive in the shade and in the tangles of thick woods. Here in upstate NY, virtually all woodlots have some wild gooseberries or currants growing in them.
Amazingly the eradication efforts were considered successful at the time. They drastically reduced the spore innoculum and thus the spread of WPBR. However, as time went on and eradicated sites were not revisited, the disease came back and spread. Today WPBR is a serious threat to high elevation five needled pines in the Rockies and Sierra Nevadas.
However new research has proved that Ribes are not the only alternate host. Indian paint brush and Snap dragons have both been shown to spread WPBR to pines.
While in the U.S., eradication and the banning of Ribes plants was the focus, in Europe the aim was to breed resistant pines and ribes. This is largely because currants and gooseberries are so popular in Europe. American nurseries and fruit growers wanting to grow ribes pushed for the legalization of resistant varieties. While countless wild plants existed all over, many states decided to permit the sale of resistant ribes varieties. And then, amazingly, in 2012 it was discovered in New Hampshire that WPBR has mutated. There is now a new strain of the fungus that overcomes the resistance in many varieties. Gooseberries and currants that have been advertised as immune by multiple nurseries across the country are actually totally susceptible to the mutated fungus.
Growing ribes has dramatically increased in popularity in the U.S. in recent years. So called ‘resistant’ and ‘immune’ varieties helped gooseberries and currants get a foothold into American gardens and farms. They are delicious, useful berries, and easy to grow. With outstanding health benefits, ribes have become a growing component of the local food movement. So what now? since there are virtually no ribes that can be guaranteed to be resistant to WPBR, is it okay to grow them?
This is a choice for individual land owners to answer. Here are some key points to consider, are there young, desirable white pines growing nearby? Are you willing to watch the pines and prune out infected limbs (an effective control measure since 99% of infections on white pines take place lower than 9 feet high)? Are there wild ribes already in the vicinity?, snap dragons or indian paint brush?
Resistance on the side of the pines is known and growing. About 20% of unselected white pines are able to fend off the disease even while young. 75% of selected pines from breeding programs are resistant. These numbers will likely only go up with time, as the genetics of the most susceptible trees are killed off.
It is interesting to know that in Czech republic and in Poland, Eastern White Pine (pinus strobus) native to the U.S. is increasingly naturalized. This is a part of the world with a high level of European black currant (ribes nigrum) production. Ribes nigrum is considered the most susceptible of all the ribes species. Black currants are not just found on a farm level, where plantings are extensive, but also at individual homes. Over 90% of homes in Eastern Europe have black currants planted. Despite this massive presence of the most susceptible ribes species, white pines have been able to naturalize themselves into the landscape.
To me, growing gooseberries and currants is a wonderful addition to the garden and to the land. Wild plants abound and have lived alongside white pines for thousands of years. I understand the controversy surrounding growing ribes, and I welcome any comments or questions.
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