Pocket Scouting Guide revised in 2015

The popular Extension publication A Pocket Guide to IPM Scouting in Highbush Blueberries has been revised (April 2015) and is again available from Michigan State University. This third edition includes information on new pests that have emerged since the previous revision in 2008, most notably Spotted Wing Drosophila.The cost is $19.00 each, with discounts for volume purchases. For more information, see: http://shop.msu.edu/product_p/bulletin-e2928.htm

 

Glyphosate herbicide injury

Glyphosate (various trade names) is a commonly-used herbicide for controlling weeds in blueberry plantings.  This product is safe and effective if used according to the label, but can cause injury to blueberry plants if allowed to contact green stems or leaves. Glyphosate injury from applications made in 2011 is now visible in some fields.  On blueberry (and many other woody plants) this herbicide can cause short, stunted shoots and strap-shaped leaves.  Again, the injury you are seeing now in spring 2012 is from applications made last season (2011), where individual shoots were accidentally hit by the spray solution.  The photo below shows blueberry stems with developing fruit, comparing normal (left) vs glyphosate-injured shoots (right).

Injury can occur on a single stem or on the whole plant, depending on what part of the bush was hit with the spray solution.  Though not a soil-active herbicide, it is possible (through misuse) to cause symptoms over the entire plant if the concentrated spray solution is allowed to soak into the ground and saturate blueberry roots,  so be sure to mix correctly, use sparingly, and only apply to the plants (weeds) you wish to control!  Do not spray so heavily that the product soaks into the ground around the blueberry roots. Wetting bare, weed-free soil serves no purpose since glyphosate has no soil activity and will only control the weeds that are contacted by the spray solution.  As always, read and follow the product label.

Glyphosate injury (left) compared to a normal shoot (right)

Evaluating leafspot effects

Late September is a good time of year to evaluate the effects of leaf spot diseases.  If most of your leaves are still attached to the bush at this time of year, then most likely flower bud formation has occurred and the crop potential for next year is good.  If, however, your leaves have already detached by the end of September, then chances are that some flower buds did not form due to premature defoliation, and crop potential for next year has already been reduced.

The image below shows three twigs from the cultivar Star, collected on 30 Sep 2011.  Star is susceptible to Septoria leaf spot caused by the fungus Septoria albopunctata. These are older twigs from the first flush of leafy growth in spring, and by now should have formed visible flower buds at the base of each leaf.

The shoot on the left is from a bush that was protected with a fungicide, retained its leaves, and has formed 5-6 flower buds.  The unprotected shoot in the middle defoliated early, and by comparison formed only a couple of flower buds at the tip.  The shoot on the far right was also defoliated early, not by leaf spots but by hurricane Irene.  It too, has only one or two flower buds.  Note that the shoot defoliated by hurricane winds still has the leaf petioles (bases) attached -- the leaves did not abscise normally, but were torn away:  This also happens (leaves missing, but petioles still attached) with caterpillar feeding damage. 

Regardless of the cause, when the leaves are taken away too early, flower buds do not form, and the potential crop is reduced for the following year -- because fewer flower buds means fewer berries.

The two main fungal leaf spot diseases that cause defoliation on blueberries in North Carolina are Septoria leaf spot caused by Septoria albopunctata, and Anthracnose leaf spot, caused by Gloeosporium minus. The two diseases have very different symptoms, as shown below:

Anthracnose leaf spot (Gloeosporium minus)

Anthracnose leaf spot, shown above, consists of large lesions that often occur at the edge of the leaf. Examples of susceptible cultivars are Pender (shown) and Duke.

 

 

 

Septoria leaf spot (Septoria albopunctata)

 Septoria leaf spot, by comparison, produces dozens of small lesions on each leaf.  The southern highbush cultivar Star (shown) is very susceptible.

By evaluating leaf drop at this time each year, growers can get a good idea of how much disease pressure they have on their farm, whether fungicide use is warranted, and which cultivars are susceptible.

Wind burn

Wind burn occurs when high winds coincide with the emergence of new, succulent leaf shoots.  The symptoms can be mistaken for insect injury, disease, drought or chemical injury.  Characteristics of wind burn are new shoots that first appear scorched and wilted, then become dried, dead and shriveled.  Older leaves with wind damage have necrotic areas and cracks at the leaf margins.  Symptoms appear immediately on new shoots in high winds.  The first photo below was taken on the day that injury occurred, while the wind was still blowing.  The second photo shows what the injury looked like one month later.

Browning of new shoots can occur very rapidly on a windy day

Wind burn one month after the injury occurred