Disease management considerations for winter-injured blueberries

Suggestions for reduced-cost fungicide programs in winter-injured blueberry fields, and disease management considerations for replanting or renewal fields.

With winter injury hitting blueberries hard in 2015, it is easy to contemplate giving up on some fields altogether. If you have decided to forego harvest because there is not enough fruit to justify harvesting, or even if you do harvest but the crop is reduced, you are most likely looking for ways to reduce inputs. You may also decide it is time to try field renewal or replanting. Therefore, I have put together several scenarios that would minimize fungicide input costs.

Field is in bad shape and of a low-yielding cultivar – use this opportunity to replant

If you remove an old field, consider if there have been virus issues in the past. If blueberry shoestring virus (BSSV) is present, you may want to check for aphids and make sure they are killed before bushes are removed to stop them from moving and spreading the virus to neighboring fields. It may be that the bush grinding process will destroy the aphids as well, in which case an insecticide is not needed.

If there have been symptoms of tobacco ringspot virus (TRSV) or tomato ringspot virus (ToRSV), it is a bit more complicated as these viruses are spread by dagger nematodes in the soil. Virus-carrying dagger nematodes can live for at least a year if not longer and could spread the virus to the new planting. In that case, Michigan State University Extension recommends having old plants tested for TRSV and ToRSV and soil tested for dagger nematodes. It is a good idea to have soil tested for plant parasitic nematodes anyway as high numbers could cause replant problems by feeding on the roots of young plants. If virus and dagger nematodes are present, there are only a few options:

  • Soil fumigation with Telone 35 to kill all soil life.
  • Seed in a grass or cereal cover crop and practice strict broadleaf weed control for at least one year and preferably two years before replanting to blueberries. This approach will reduce nematode numbers, but more importantly, will disrupt the virus life cycle. The virus cannot live in grasses, but can survive in dandelions and other broadleaf weeds.
  • Another option, if only ToRSV is present, is to replant with the cultivar Bluecrop, which appears to be tolerant to ToRSV, but not TRSV.

When replanting, buy certified virus-tested planting material to avoid bringing in viruses in the planting in the first place. This will be well worth the higher price for the plants. Remember that tissue culture in and of itself is not a guarantee the plants are virus-free, they have to be certified by lab-testing the mother plants.

Field is in bad shape – mow off all canes and allow to renew

Remove as much of the old and diseased wood from the field as possible and burn it. In this case, we should aim to protect new growth from diseases and make sure not to push canes too hard with fertilizers so as not to predispose them to fall frost injury or infection by Phomopsis. If the field had a bad case of Phomopsis, hopefully most of the inoculum has been removed with the old canes, but one or two fungicide sprays (for instance with Quash, Indar, Tilt, Phostrol or Pristine) may be helpful to protect new growth from any remaining spores lurking in cane pieces. These fungicides are best applied during big flushes of growth and before rainy periods. Since there will be no fruit, the pre-harvest intervals are not of concern.

For cultivars that are susceptible to leaf rust, which may result in early defoliation, scout frequently and keep an eye out for rust pustules from mid-July through September. As soon as the first rust pustules are seen, a Pristine (excellent) or Indar/Quash/Tilt (good) spray can be applied to prevent further infection. This works well if you scout frequently. In organic fields, Serenade or Double Nickel 55 can be used for leaf protection; adding a sticker-extender can improve efficacy of these products.

Field has low crop – will not be harvested

Since no fruit will be harvested, protection for fruit rots is not needed. While growers may be concerned with build-up of inoculum, they should be able to take care of that with one or two dormant sprays and a diligent fungicide program next year. Therefore, the main concerns this season are with cane diseases and possibly leaf rust. If there is a lot of Phomopsis cane blight, prune out and burn diseased and dying canes to remove as much inoculum as possible. If you can’t remove them from the field, make sure to chop the canes into small pieces to speed up decomposition. Protect new growth during flushes of growth at least once and preferably twice with effective fungicides, such as Indar, Quash or Tilt. Pristine is also effective, but is probably too expensive in this scenario. Phostrol or another phosphite fungicide may also be helpful; these products are highly systemic and work like Aliette.

For cultivars that are susceptible to leaf rust, scout frequently and keep an eye out for rust pustules from mid-July through September. As soon as the first rust pustules are seen, a Pristine (excellent) or Indar/Quash/Tilt (good) spray can be applied to prevent further infection. This works well if you scout frequently. In organic fields, Serenade or Double Nickel 55 can be used for leaf protection; adding a sticker-extender can improve efficacy of these products.

Field has moderate crop – will be harvested

In this case, you would follow your normal program, but using fewer sprays (only apply at critical timings) and use less expensive products. Critical timings after bloom would be early green fruit, first blue fruit, during flushes of new shoot growth and right after mechanical harvest. You can also use lower rates, but this is advised only for protectants that are not vulnerable to fungicide resistance development. The least expensive products for blueberries are sterol inhibitors (e.g., Tilt, Indar, Quash: $12-18 per acre), phosphites like Phostrol ($7-18 per acre), copper products ($5-15 per acre), Captan ($7-18 per acre) and Ziram ($10-14 per acre). You can further save by buying generic products and by adding adjuvants that can improve coverage and retention. Scout fields frequently to look for disease symptoms and rust.

Dr. Schilder’s work is funded in part by MSU’s AgBioResearch.

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