April 1, 1998
Compiled by Vern Grubinger, University of Vermont Extension
(802) 257-7967 verng@sover.net

Blueberries evolved under acidic soil conditions where levels of many nutrients are naturally low. Generally, bushes require relatively small amounts of most nutrients, and are sensitive to too much fertility - if a little is good, a lot is not better! Most soils can supply adequate quantities of nutrients (except nitrogen) if the pH is maintained in the proper range. Soil testing is a valuable part of blueberry nutrition programs. Soil pH needs to be monitored because pH influences the availability of many nutrients to plants. Sample soil from existing plantings every 3 to 4 years. All plantings may be sampled the same year or portions may be sampled more frequently on a rotating basis. Collect soil from within the row to a depth of 8 to 10 inches. The vast majority of roots are usually in the top foot of soil, and rarely do blueberries root deeper than 16 inches. Sample at any convenient time of year.

Soil sampling is particularly important where the soils have been acidified. If the acidifying agent was applied to the soil surface and not incorporated, pH will likely increase with depth. Collecting separate topsoil (0 to 8 inches) and subsoil (8 to 16 inches) samples will provide a better understanding of whether the soil has been acidified adequately. Acidified soils often increase in pH over time so these plantings need to be monitored more frequently than sites on naturally acidic soils.

Soil tests also provide estimates of the quantities of P, K, Ca and Mg that are available to plants. Soil tests, however, provide only crude estimates of nutrient availability to plants, and should be complemented by periodic leaf tissue analysis. Leaf analysis is a valuable and often under- used tool in blueberry nutrition programs. It provides a means of accurately identifying nutritional problems that are difficult to diagnose by soil testing or observing bush appearance, and can help you identify and correct potential nutrient problems before growth or yield is affected.

If possible, submit different tissue samples for different varieties, blocks and/or soil types. Sample bushes every 3 to 4 years. Each sample should consist of 50 to 100 leaves collected from current season shoots taken from bushes throughout the sampling area. Mark you calendar now with a reminder to collect leaves in early to mid-August!

Nitrogen fertilizers that supply N in the ammonium form are preferred over those supplying nitrate?N, because nitrate may injure blueberries. Urea is generally the best choice if the soil pH is less than 5.0. Urea is high in analysis (46% N), is an inexpensive source of N, and is considered an ammonium source. If the soil pH is somewhat high (above 5.0), ammonium sulfate is the best choice. Though urea and ammonium sulfate tend to decrease pH, ammonium sulfate is more acidifying. If complete fertilizers (containing N, P and K) or blends are used, make sure the majority of the N is in the ammonium form. Note: ALUMINUM sulfate is not a fertilizer, but rather an acidifying agent.

Mature blueberry plantings need 50 to 70 lb N/acre annually, but rates must be adjusted for soil type and site. Plantings on sandy soils low in organic matter will need higher rates; those on heavier soils high in organic matter require lower rates. Plantings mulched with sawdust or wood chips may need up to double the application rates recommended, depending on the type of mulch and method of application, because mulch materials tie up N, making it unavailable to the plants. Coarse mulches and targeted application (ie through drip under mulch) lessen the need to increase rates.

Apply N between bud break and petal fall in the spring. Earlier applications may be wasteful because bushes cannot absorb soil N readily until leaves begin to grow. Blueberries are shallow-rooted and can absorb N quickly. Multiple applications will often increase the efficiency of N use, particularly on sandy soils where N can easily leach out of the root zone before bushes can use it. Split the annual rate into 2 or 3 equal portions and apply these in May and June. Do not fertilize plants later than July as it may stimulate late growth that is prone to winter injury. It is possible to supplement annual soil applications of N with foliar sprays during the season. Supplemental N sprays may benefit N-deficient bushes, but bushes receiving appropriate soil applications of N are unlikely to show a yield response to sprays.

EARLY SEASON GRAPE PESTS (grape production is on the rise in Vermont, so I'll be providing more information on this crop than in years past...)
The grape flea beetle is sometimes a serious pest of grapes. Adults are metallic greenish blue or steel blue beetles less than 1/4 inch long, and they emerge from overwintering and move into grapes as buds swell in April. They chew holes into the tips and sides of buds and destroy the buds' potential to produce primary or secondary shoots. After buds have grown to a length of ½ inch or more, the beetles rarely cause significant injury. Later in the season larvae and adults feed on the lower surfaces of leaves, but they rarely cause any real injury. Scout for flea beetles as buds swell in April and May (and pay particular attention to edges of vineyards); applications of Sevin (carbaryl) are justified if more than 4 percent of the buds are damaged. For organic growers, rotenone gives some control of grape flea beetles.

Climbing cutworms also damage grapes early in the spring. Problems are often greatest in weedy areas of vineyards and in plantings on sandy soils. Look for cutworms and their damage as buds swell ?? scout at least twice weekly. Applications of Sevin (80S or 50WP) are effective against climbing cutworms on grapes.

To control European Red mites and/or scale insects, apply Superior oil at 2 gallons/acre before buds show any green tissue. Always follow the pesticide label instructions.

Copies of the New England Vegetable Management Guide are now available from my office for $7.50 including postage. The New England Small Fruit Management Guides have been printed, and should be available next week for $6.50 postage included. Membership in the Vermont Vegetable and Berry Growers Association entitles you to one free guide per year. If you are not a member, or need another guide, please call my office at (802) 257-7967.