|Ribes Species and White Pine Blister Rust – An Update, June 9, 2015
Over the past several years, interest has increased throughout the Northeast in growing and cultivating currants, gooseberries and other species in the genus Ribes for backyard and commercial fruit production. Stimulated by development of varieties that were either resistant or immune to the white pine blister rust pathogen, Cronartium ribicola, several states, including New York and New Hampshire, have eased the once-standard and universal quarantine of Ribes plants that protected the white pine resource and that had been in place for many decades. Unlike other neighboring states, Maine never changed its Ribes quarantine law, and now that decision has come to work to our advantage. A new strain of the pathogen C. ribicola, identified in late 2010, is now known to be able to infect previously resistant and immune species and cultivars of Ribes.
A study was completed in 2014 by the USDA Forest Service to determine the effects of this new strain of C. ribicola on Ribes and the white pine hosts in New Hampshire. The presence of C. ribicola was confirmed on 17 of the 19 immune or resistant Ribes cultivars screened. The study also reported an 18 percent probability of finding white pine blister rust on pines neighboring black currants that were infected with the new pathogen strain but only a 2 percent probability of finding the rust on pines neighboring pathogen-free Ribes. The difference was highly significant both statistically and epidemiologically. The full report appears athttps://apsjournals.apsnet.org/doi/abs/10.1094/PDIS-12-14-1338-RE.
Results from this study show that the breakdown of resistance in Ribes poses a significant threat to the white pine resource and to cultivated Ribes production. For this reason, all Maine farmers and gardeners need to be aware that the state quarantine prohibiting the culture of European black currants (Ribes nigrum) and all its cultivars throughout the state, and prohibiting European black currants and all other Ribes spp. in the defined quarantine zone, remains in effect. The map and town list of the Maine quarantine areas can be viewed at
William D. Ostrofsky, Forest Pathologist
By Roberta Bailey
Currants, gooseberries and jostaberries, a hybrid black currant-gooseberry cross, are making a comeback. Interest in growing ribes species is growing in spite of the present ban on currants and gooseberries in Maine. (See the end of this article for more information.)
Currants are extremely hardy, productive, fairly disease resistant, and ornamental plants. Traditionally used in making jams, jellies and juices, they also have a wide range of herbal uses, from a curative for mouth and throat ailments, fever and pregnancy ailments, to a deodorant and a promoter of longevity. When black currant twigs are boiled in water for 20 minutes to make a tea and the tea is taken daily, it can significantly reduce or eradicate severe excema. Currants are high in vitamin C, B and A, as well as several minerals.
Red currants (Ribes rubrum) are tart, yet very flavorful, the source of the full-bodied red jelly that is commercially available. White currants (Ribes sativum) are less acidic and sweeter and are usually eaten fresh. They are not the source of the lightly colored, dried ‘currants’ in stores, which are actually a small grape. Black currants (Ribes nigrum) make excellent liqueurs, jams, fellies and herbal remedies. Many cultivars are excellent eaten fresh. Black currants have five times as much vitamin C as oranges.
Native to cool climates, the earliest currant seeds were found in a Danish Mesolithic site. They grow well in New England and range from zone 2 to 5. Extended exposure to temperatures over 86 degrees may cause leaf loss.
Since the plants start to leaf and bloom quite early in the spring, choose a site that protects against late spring frosts. Currants thrive in full sun but will produce in partial shade. They are undemanding in their soil requirements, growing on sites with as little as 1 percent organic matter. The ideal soil is a fertile loam enriched with compost or well-rotted manure with a pH between 5.5 and 7,0. Currants are shallow rooted and need a moist soil. They can tolerate heavy soils but will not grow on a wet site.
Traditionally, black currants were self sterile, needing two varieties planted close together to pollinate each other. Modern black currant varieties are self fruitful. All other currants, as well as gooseberries and jostaberries, are self fruitful and can be planted individually or as a single variety row. Two varieties will increase yields slightly.
Currants are borne in clusters. They bloom over an extended time – up to 20 days between the terminal bud and the basal flowers. Black currants are borne in small individual dusters. Choose varieties with overlapping bloom times.
Red and white currants will grow to be 3 to 5 feet in height and width. Black currants at maturity are up to 6 feet tall and wide. Space plants at least 5 feet apart with 8 to 12 feet between rows.
Plant very early in spring or in September, when plants have plenty of time to establish roots and be ready for early spring growth. Plants are sold bare-root or in pots. Dig a hole to accommodate the existing root system plus future growth. Set plants slightly deeper than they were in the nursery. Older or larger bushes should be set so that the lowest three buds can be buried. Fill the hole with compost enriched soil and tamp the ground to remove air pockets. Clip the branches to five buds in length.
Vegetatively propagated plants form their roots in the top 8 to 16 inches of soil, (virtually all commercial plants are vegetatively propagated from cuttings) and seedlings have a small taproot. All benefit from a 6-inch mulch of bark or wood chips. Because currants are shallow rooted, soak the soil thoroughly and mulch the plants 5 to 6 inches with wood chips or organic matter to retain moisture and keep the soil cool. Water the plants two to three times a week for the first month, then see that the plants get 1 inch of water per week for the first year.
Pruning and Fertilizer
To maintain the best production, occasional pruning is necessary. Prune when plants are dormant. Black currants fruit on one-year-old wood and spurs of two-year-old wood. Once they are established, remove one-quarter to one-third of two-year-old wood each year. Poorly growing bushes can be revived by cutting weak or old growth to ground level. Identifying the age of growth is simple: New shoots are pale tan, second year wood is gray, and older wood matures black.
Red or white currants bear on two- and three-year-old wood. Prune to remove older wood. New shoots emerging from the base of the plant should be thinned to the strongest six.
Currants benefit greatly from regular feeding, especially of nitrogen and potassium. A three-year-old plant will use 2 ounces of nitrogen per year, a four-year-old uses 4 ounces. Feed annually with a complete organic fertilizer and compost. Bone meal and sul-po-mag are slowly available sources of nutrients, but should not be relied on as the only sources.
Fruiting will occur one to two years after planting. Full production is attained in the fourth or fifth year, with black currant plants producing for at least 10 years and red and white bearing for 15 to 20 years. Fruit ripens over an extended period in June through August, with black varieties maturing first, followed by red, then white. A three-year-old bush yields 2 to 5 quarts. A mature red currant bush yields 5 to 10 quarts. Yields of black currants can be higher.
One of the graces of the fruit is that it hangs on the plant for a week or more after ripening, giving the busy gardener some leeway in harvesting. Red and white currants form on a sprig and can be picked on the sprig at one time. Black currants grow as individual berries and are best harvested over several pickings, as the flavor improves with maturity. Slightly underripe fruit yields the highest pectin levels for jelly making, while dead ripe fruit lends full flavor to juices.
Small, isolated plantings of currants tend to be relatively disease resistant. Aphids, scale and borers occasionally attack currants but do not cause serious damage. The currant fruit fly can be a problem: The larvae bore into the fruit after egg hatch and the fruit discolor prematurely as they are eaten inside by the larvae. Bacillus thuringiensis is an effective control. Imported currant worm defoliates plants in some years as it feeds on leaves. Check the undersides of leaves to detect them. Rotenone or pyrethrum can control them (although rotenone can be quite toxic to the applicator).
Powdery mildew and anthracnose occasionally plague currant bushes. Powdery mildew, a white powdery growth on the leaves, branches and fruit, is the most common disease you will encounter. To avoid both, choose resistant varieties, don’t crowd plants and keep plants pruned to encourage good air circulation and reduce the the spread of fungal spores. Anthracnose discolors and forms spots on leaves.
White pine blister rust is the infamous disease that keeps most Mainers from being able to grow Ribes. Because wild Ribes and some cultivated Ribes species and varieties host Cronartim ribicola, a parasitic fungus that causes white pine blister rust on any five needle pines (white and stone pine), states with pine lumber interests outlawed the import of all Ribes in the early 1900s. Maine is the only state that still has an active eradication program in place, though a handful still disallow import of the plants.
The situation is fairly complex. In spite of restrictions, white pine blister rust persists. Climate is the most important factor in the spread of the disease: It is spread more in years of moist weather, less in dry years. This happens regardless of eradication programs. Spores from pines travel hundreds of miles and infect Ribes, and spores from Ribes travel tens of miles or more to infect pines. Susceptible Ribes species are wild throughout the United States arid Canada. Maine hosts 40 wild species of Ribes. Some Ribes appear to be resistant or immune, and breeding and testing is being done on new strains to promote this trait.
Some question whether immune or resistant varieties spread to the wild would carry those traits with them. Others suggest that so many wild plants are being spread that planting resistant varieties would introduce more resistant genes into the wild as birds spread seed that may contain resistant genes.
Pines have some resistance: white pine blister rust could not wipe out the pines. Foresters are screening for and propagating resistant trees. Research is being done on spraying a low grade mineral oil on Ribes (during the uridinlal stage) to prevent or reduce the spread of the disease.
In addition, the death of pines from white pine blister rust seems to involve other fungi. Research is being done in this area as well.
As it stands in Maine, Ribes nigrum is banned. Red currants and gooseberries are banned in most counties, but can be grown in parts of Oxford, Franklin, Somerset, Piscataquis, Penobscot, Aroostook and Washington counties. (See box,)
Pressure is being put on many states to change the laws because many people are interested in Ribes cultivation. Ocean spray now produces a cranberry-currant juice. Let our Agriculture Department know that we want to be able to grow this fruit.
The Backyard Berry Book, Stella Otto, Ottographics, 1995.
Food, Waverly Root, Simon & Schuster, 1980.
Letter from Kim Hummer, USDA Ribes expert, Corvallis, Oregon, January 6, 1999.
Ribes researcher with USDA; Kim Hummer (email: [email protected]) USDA ARS NCGR-Corvallis, Oregon
TIRA, The International Ribes Association; Box 428, Boonville, CA 95415, Email: [email protected]_net. Membership $20.00/yr
White Pine Blister Rust (WPBR) Information Page (www.cce.cornell.edu/columbia/smckay/wpbr.html). Web Editor: Steven McKay, founder of TIRA, The International Ribes Association. Email at: [email protected] CCE of Columbia County, 479 Rte. 66, Hudson, NY 12534. (518) 828-3346. Vision Statement: Both white pine and ribes should be able to grow as commercial crops without economic damage from white pine blister rust
[Cornell Fruit Resources: Gooseberries and Currants; www.fruit.cornell.edu/mfruit/gooseberries.html]
Ann Gibbs, State Horticulturist, Maine, Dept of Ag., 28 State House Stn., Augusta, ME 04333; (207) 287-3891; (207) 287-7548; [email protected]
Dave Struble, State Entomologist, Maine Forest Service, 22 State House Stn., Augusta, ME 04333; (207) 287-4981; dave.strub[email protected]
Clark A. Granger, Forest Pathologist, Maine Forest Service, 50 Hospital Street, Augusta, ME 04330; (207) 287-2431; [email protected]
William D. Ostrofsky, Assoc. Research Professor, University of Maine, Cooperative Forestry Research Unit, 5755 Nutting Orono, ME 04469-5755; (207) 581-2877; [email protected]
Current Ribes Law – State of Maine
The White Pine Blister Rust Regulations and Quarantine are listed under Title 12 MRSA1988, Subchapter III, Section 803:8305 Shipment Prohibited.
The director may prohibit, prevent or regulate the entry into or movement within the State, from any part thereof to any other part, of any plants of the genus Ribes or other nursery or wildling plants, stock or parts of plants which may cause the introduction or spread of a dangerous forest insect or disease. The director may issue the necessary orders, permits and notices necessary to carry out this section which shall not be considered to require or constitute an adjudicatory proceeding under the Maine Administrative Procedure Act, Title 5, Chapter 375.
Regulation: White Pine Blister Rust, Quarantine on Currants and Gooseberry Bushes.
A. The sale, transportation, further planting or possession of plants of the genus Ribes (commonly known as currant and gooseberry plants, including cultivated wild, or ornamental sorts) Is prohibited in the following Counties in the State of Maine, to wit: York, Cumberland, Androscoggin, Kennebec, Sagadahoc, Lincoln, Knox, Waldo, Hancock, and parts of Oxford, Franklin, Somerset, Piscataquis, Penobscot, Aroostook, and Washington.
B. The planting or possession of European Black Currant, Ribes nigrum or its varieties or hybrids anywhere within the boundaries of the State of Maine is prohibited. This quarantine is administered by the Insect & Disease Management Division of the Maine Forest Service.