Blueberry Council of Missouri

 

Blueberry Council of Missouri Newsletter


The Importance of Disease Prevention and Management in Blueberries

By Donna Aufdenberg, Regional Horticulture Specialist, Southeast Region, University of Missouri Extension
October 2009

In mid-summer, I received several calls from blueberry growers complaining about poor yield, poor growth and dieback. My first response when I have a call like this is “Have you had a soil test lately?” and most of the time a soil test is due. More times than not, these symptoms tend to be related to soil pH issues, a nutrient deficiency or imbalance, and even cultural practices or lack thereof. However, knowing the history of our past weather events, we have to take a close look at whether diseases have played a role in the symptoms.

After several site visits, I found myself questioning what a normal blueberry plant looks like. I never had to think about it until receiving a diagnostic report back on a plant sample that had been sent up to the University of Missouri’s Plant Diagnostic Clinic. The report confirmed Botryosphearia stem canker, a fungal disease. The grower and I took the report with sample pictures into the blueberry patch to identify the disease and we came back more confused than ever. I visited with another grower and sent more samples to the diagnostic lab and the results came back diagnosing Phomopsis canker, another fungal disease. So we started asking "How much of this is widespread in the blueberries across the state?"

So what are we seeing that concerns us? The first indicator is the dieback - the dead stems or "flagging" that is seen on random plants. Then there are the living stems that have no leaves but still have berries long after harvest. These plants look very ragged. If you look across the patch from variety to variety, you can see some varieties are thriving with lots of growth while others appear stunted, no new shoots at the base of the plant. The ailing varieties tend to be Blue Crop, Blue Ray, Blue Jay, and Duke just to name a few. When looking at the struggling or dying canes, you find raised red to rust lesion-like areas on the newer growth and cankers on the aged wood at the base. The raised areas on the younger stems sometimes look mottled. There are some tan to reddish spots that are centered around buds. On some of the more severe canes, there are sunken and withered areas. Foliage on these plants tends to reddening and/ or yellowing and sparse on branches.

Some growers and specialists suspect that these problems go back to the spring freeze of 2007 which damaged so many different species of plants. For blueberries, it did ruin one year’s crop but could it have left blueberry plants susceptible to a whole host of problems? Did the overcropping on some of the varieties in 2008 stress the plants too much making them prone to more problems? Then we have to look at this year’s spring and summer weather. Super wet and cool temperatures state-wide increased disease pressures on many different crops. So why would it be any different for blueberries?

For several growers, cultural practices were also in question. Were the plants healthy enough to ward off these potential diseases? Cultural practices such as pH management, fertilizing, irrigating, pruning and disease prevention sprays are crucial for blueberry health.

  • It is very important to take soil tests to determine pH and nutrient levels. Taking one every year is the best way to keep track of any changes!
  • Do you fertilize according to a soil test and supplement based on blueberry production recommendations? Do you irrigate wisely? These things are important for healthy plants!
  • Do you prune correctly? Do you prune enough? Pruning is needed to remove unproductive canes, diseased canes and promote new growth. The best time to prune is during late winter or early spring just before bud swell.
  • Disease Prevention should be at the top of everyone mind for next year! Are the blueberry growers who did not spray in spring to prevent diseases the ones most severely hit? That is a good question. Up until now, many growers have not felt a need to apply preventative sprays. If you have not been spraying, consider developing a spray schedule.

Now, after making those points, I work with one grower who is a great manager and he concludes that with this year, it sure didn’t matter what you did or didn’t do, they just looked bad. He says “If I would have known what my plants would like 2 years after the spring freeze of 2007, I would have leveled all the bushes to the ground that spring!” He pruned back some varieties back to the ground in 2007 that were most severely affected by the freeze and they are looking great. Hopefully he will have some good blueberries on those.

So, what could be affecting our bushes? The main blueberry stem diseases that are reported in the “Growing Blueberries in Missouri” Bulletin published by Missouri State University include:

  • Botryosphaeria stem blight caused by Botryosphaeria dothidea
  • Botryosphaeria stem Canker caused by Botryosphaeria corticis
  • Phomopsis Twig Blight caused by Phomopsis vaccinii
  • Fusicoccum Canker caused by Godronia cassandrea

For specific information on each of these diseases, see the “Growing Blueberries in Missouri” Bulletin located at http://mtngrv.missouristate.edu/Publications/GrowingBlueberries.pdf

Stem and twig blight affects individual canes or stems on the blueberry plant, with one or more canes dying each year until the entire plant dies. Canes with this lose vigor quickly and leaves turn yellow to red. The internal tissue exhibit a tan discoloration and marks where the fungus invades the wood.

With all canker related diseases, an area where the fungus grows is marked by swollen lesions that persist and become larger each season. The cankers disrupt the flow of water and mineral elements to the tips of the stems and branches. Most growers notice sudden wilting and death of leaves on infected stems. Stems turn a red to brown color. Most of the diseases overwinter on cankered wood, so it is important to inspect and be able to identify infected areas.

Disease spores spread generally in spring and are spread via the wind or by rain splashing. The spores can infect new growth on nearby plants as long as there are rainy periods and moderate temperatures. Mechanical damage by equipment and pruning during the growing season can increase disease incidence also.

What are the best management practices? Whether you suspect your blueberries have one of these diseases or not, it is always wise to take these precautions and take preventative measures.

  • If purchasing new plants for expanding your operation or for replacing plants, buy only disease-free nursery stock from reputable sources.
  • When pruning in the dormant season or working in the patch during the growing season, avoid mechanical injury to stems and branches that might provide an entry point to pathogens.
  • Avoid stressing plants during the growing season. If you don’t irrigate, start! Adequate moisture is very important. Most of the blueberry roots are within the top 6-8 inches and dry out easily. Tensiometers are a wise decision for any grower who wants to take the guesswork out of when to irrigate. Also, mulching helps conserve moisture.
  • Prune out and destroy diseased canes before next year’s growing season and before they have time develop further and spread more disease spores. Take care to remove all infected tissue below the cankered or infected area. Destroy by burning canes and branches. Use sanitation practices when pruning. Sanitize pruners with 10% bleach solution between cuts or dip them in full strength Lysol to disinfect pruners. Lysol is less likely to corrode the blades than bleach.
  • Use dormant sprays of lime sulfur or Sulforix to help reduce inoculum. Use Captan or Ziram 75DF when leaf buds are showing 1/16-1/4 inch green tip. (according to the 2009 Midwest Commercial Small Fruit and Grape Spray Guide)
  • Scout the blueberry plant regularly! With some growers, once harvest is done, they don’t re-enter the patch until pruning time. How do you know what is happening to your plants unless you look at them? Scouting is sometimes the best method for heading off problems because you can catch issues before they become real problematic!
  • When in doubt, contact someone who can help whether that is a University of Missouri Extension Specialist, an industry specialist or another knowledgeable grower.
  • Send suspect plant materials to be confirmed by University of Missouri Plant Diagnostic Clinic, 23 Mumford Hall, Columbia, Missouri 65211 Phone 573-882-3019