Blueberry Update 2016

There’s been plenty of blueberry rust in 2016, likely as a result of a milder than average winter and the heavy rainfall observed in many blueberry production areas. In some fields we’ve observed active sporulation of rust on old leaves from 2015 that have not abscised. The blueberry IPM guide at www.smallfruits.org provides information on specific fungicides for rust management.  If a variety is susceptible to rust, early defoliation can result, and this will reduce yields next year as a result.  Scouting is required to determine whether rust is developing in specific varieties.  Many fungicides applied for other leaf spots will also have rust activity; this is particularly true of the strobilurin (Abound, Pristine, etc.) and DMI fungicides (Proline, Quash, Tilt, Orbit, Indar, Bumper, etc.). Growers need to manage post-season leaf spot, and consider fungicide applications as needed to prevent premature defoliation due to rust, Septoria, and anthracnose leaf spots. For organic growers, Sonata is one of the few organic fungicides that lists rust on the label.

Blueberry Rust Research Trial

We began a research trial looking at different rust fungicide treatments in blueberries with Dr. Phil Brannen and Grad Student, Russell Ingram. Four county agents and I have started putting out these fungicide sprays in our counties.

Over the past few years, rust on blueberries has become an increasingly problematic disease. Recent informal reports have indicated that the disease is occurring with an increased prevalence and earlier occurrence in South Georgia than in past years. The exact reason for this is currently unknown, but the implications are obvious. Blueberry plants suffering from high levels of rust disease often defoliate premature. While defoliation post-harvest obviously has no implications on fruit production within season, it does have significant implications on yield and plant health the following years.

The causal agent of blueberry rust, Pucciniastrum vaccinii, has no known alternate host (Tsuga sp.) in Georgia outside of the mountains in the north. There are two possible sources of primary inoculum that could be causing annual outbreaks of disease. The first is locally dispersed inoculum from infected leaves from the previous year still retained on the bush. This hypothesis explains the buildup of disease and the earlier occurrence that has been observed over the past few years. The second possibility is regionally dispersed inoculum blown in from commercial blueberry production in North Florida. This hypothesis is equally as likely as the first because the earlier occurrence of the disease could be a result of the increased use of early developing varieties used in Florida.

In the case of locally dispersed inoculum, fungicides (i.e. Sulforix) that reduce the amount of retained infected leaves and reduces the amount of overwintering inoculum will likely provide the greatest level of control. This type of control strategy will be exceptionally important following a mild winter such as that experienced from 2015 – 2016. In the case of regionally dispersed inoculum, fungicides (i.e. Bravo) that provide extended periods of protection both pre and post-harvest will likely provide the greatest level of control. The optimum control strategy would be to incorporate the reduction of carry-over inoculum with in-season protection by employing a pre-harvest material (i.e. Pristine or Abound), a persistent post-harvest material (i.e. Bravo), and a fungicide that induces defoliation as well as eradication of surface inoculum (i.e. Sulforix).

The purpose of this research trial is to evaluate the efficacy of eight individually applied materials and one interval spray utilizing two different materials for controlling rust on blueberries in five counties in South Georgia. This is but one component of a comprehensive rust-management program, and other components will be addressed in the future.

Plots are randomized, replicated 5 times and treated every two weeks. We will be working on this trial March 2016 – September 2016. The results will be reported when completed. Leaf samples will be taken regularly and Russell Ingram told me he will be evaluating over 10,000 blueberry leaves for this research trial! The results will give us a much better understanding on how to manage rust.

Anthracnose

Anthracnose in blueberries has been a concern this year as well. Unfortunately, if producers have not sprayed well up to this point on southern highbush blueberries, the disease may already there for the season. There is also the possibility of resistance to strobilurin fungicides (Pristine, Abound, etc.). If there is confirmed strobilurin resistance then fungicides that contain the active ingredient trifloxystrobin, pyraoxystrobin, pyrametostrobin, pyraclostrobin, picoxystrobin, azoxystrobin, dimoxystrobin, fluoxastrobin, metominostrobin, or orysastrobin will not be effective. Only a few of these are even labeled in products for blueberries. I just listed them for informational purposes.  All of the strobilurin fungicides are a FRAC code 11 in the IPM guide. So, if you use Abound, Pristine, and Quilt Xcel, those all are strobilurin chemicals. Based on what some producers have sprayed I think resistance is highly likely, although other things can cause disease problems. We have confirmed stobilurin resistance in strawberries in Appling county, but not yet in blueberries. I will be sending samples off from fields around the county and report on those results at a later date. In the meantime, stay on a good chemical rotation and use another mode of action after two sequential sprays with a strobilurin fungicide.

Nematodes and replant disorder

We took nematode samples at 5  different farms that had been replanted, or about to be replanted in Appling County. Ring nematode is the primary concern, but all nematodes could cause problems in a replant scenario. 7 out of 12 samples in Appling county had high levels of nematodes. Unfortunately, all but two of these sites have already been replanted making fumigation impossible. These replanted plants will likely die out within 3 years and the site will have to be fumigated and replanted again….very expensive.

On a perennial crop like blueberry, plant-parasitic nematodes can build up year after year, sometimes for a decade or more until population levels are high enough to cause damage. Symptoms of nematode damage include stunting, yellowing, shoot death and eventually plant death. If a grower plants young blueberry plants back into a nematode-infested soil, damage in the replanted area may be immediate and severe – replant disease.

If the farm is infested with ring nematodes, then the grower could lose the entire investment at about the time that the blueberries would normally be coming into production. It is possible to delay the onset of blueberry replant disease by application of soil fumigants, but eventually the ring nematode will come back, and plant vigor will suffer, thus shortening the life of the planting. At this time there is no post-plant nematode control method available for blueberry.

Below are results from a blueberry nematode research trial in Appling county:

There are several pathogenic nematodes present in most samples, any of which might cause problems in a replant scenario. It is recommended to apply an application of PicChlor 60, Trifecta or the Georgia 3-Way (vapam, chloropicrin, and Telone) in a blueberry field being replanted. For replants, we have collected enough data now that we realize the death and decline of replant disorder is broader than just nematodes, so Telone alone is not recommended, though it would control the nematodes and plastic would not be required.

Note: Don’t confuse replant disorder and nematode damage to bacterial leaf scorch. In a new field planted behind cut over timber it is highly unlikely that nematodes are a problem. However, we are seeing bacterial leaf scorch in susceptible varieties. There is also Botryosphaeria and Phytophthora root rot which can cause plant dieback as well. Ridomil Gold and/or 4 applications of a phosphonate fungicide (Aliette, ProPhyt, AgriFos, K-phite and others) will work well for Phytophthora root rots in the first year after transplanting.

Some producers are already picking and it won’t be long until harvest is full swing!

Contributors:

Phil Brannon

Russell Ingram

Ted Holiday