Editor’s Note: This is the third article in a three-part series covering blueberry-related educational sessions from the Southeast Regional Fruit and Vegetable Convention in Savannah, GA, in January. Thanks to SBI Industries, manufacturer of TotalGro water soluble fertilizer, for sponsoring this month’s report.
Rabbiteye cultivars continue to dominate commercial blueberry production in Georgia, although more and more acreage has been devoted to southern highbush varieties. Recently, new rabbiteye and southern highbush varieties have been introduced, and due to superior characteristics, producers are replanting established blueberry sites.
With this replanting has come new discussions concerning fumigation. Fumigation, though practiced with many fruit commodities such as strawberries and peaches, had not been a recommended practice for new establishment of blueberries. However, it is not known whether fumigation would be of benefit in blueberry replant sites. Information has been sketchy and anecdotal at best.
An initial trial was conducted by University of Georgia (UGA) researchers in 2007 in Alapaha, GA, to review replant disorder in blueberries. At that point, replant disorder, if it existed, was assumed to be related to fungal organisms, especially Phytophthora and Pythium species. However, the results in this trial indicated that only the fumigants impacted the plant health of replanted blueberries. The fungicide Prophyt (phosphonate, Helena Chemical) did not impact the replants in any way, so oomycetes were not of major significance in this trial site.
|The chart below indicates survey results of plant-parasitic nematodes in commercial blueberry plantings in Georgia in June 2010.|
|Nematode Species||Percent Frequency*||Mean Density/100 cm3 soil**|
|Ring (Mesocriconema ssp.)||48||290|
|Stunt (Tylenchorhynchus ssp.)||8||18|
|Stubby Root (Paratrichodorus ssp.)||8||13|
|Lance (Hoplolaimus ssp.)||7||130|
|Sheath (Hemicycliophora ssp.)||6||46|
|Awl (Dolichodorus ssp.)||2||19|
|Dagger (Xiphenema ssp.)||2||16|
|* = Percent of total samples with species present, N=289 samples from Appling, Atkinson, Bacon, Berrien, Brantley, Clinch, Cofee, Jeff Davis, Lanier, Pierce, Ware, and Wayne Counties
** = Mean population density for samples with nematode species present
In light of this data, UGA nematologist Jim Noe suggested checking for nematodes, and the results were interesting. Nematodes were previously not thought to be an issue on blueberries, and in fact, most sources of information indicated no or limited nematode activity in blueberries.
A closer look at ring nematodes indicated that perhaps more research was necessary. Ring nematodes obtain their name from the deeply striated cuticle. Various ring nematodes (Mesocriconema species) are known to cause damage on other fruit commodities, but the association with blueberries has never been established. However, large numbers of these root feeders could possibly be of importance, the researchers noted, due to direct activity and causing damage and stress for other pathogens.
Creating A Link
With this in mind, UGA researchers set out to confirm the importance of replant disorder in blueberries, and address nematodes as associated with replant disorder. Field fumigation trials were established at two sites (Alma and Homerville, GA) with five different treatments. Data was collected related to plant vigor and survival. Nematode data was also collected from each site, and ring nematodes were consistently associated with these sites.
Initial results indicated substantive benefits in vigor and plant health of plants following fumigation, especially as related to decreased ring nematode populations. The nematode counts at Alma were much less than those found in the Homerville site, and the degree of the fumigant effect was also greater at the Homerville site. At both locations, PicChlor 60 (60% chloropicrin, 40% Telone) provided good management of nematodes. A lower rate of Telone II was not sufficient for strong control of ring nematodes in these trials, though this rate will often control nematodes. At both sites, the untreated plastic only control did provide a vigor increase. It is presumed that there is some solarization effect involved in this, but this is not fully understood.
A subsequent survey of nematodes in blueberries in July 2010 indicated that ring nematodes are prevalent in replant sites. Ring is consistently a very high count nematode species, but others, such as lance, are also high count where they occur. However, ring was found in half of the blueberry sites in south Georgia.
In addition to noting the link between ring nematodes and stunting with replant disorder, the researchers also evaluated reproduction rates of ring nematodes under greenhouse and field conditions. It is important to note that these were short-term studies, and high populations of nematodes actually crash, whereas low populations build up. The bottom line, however, is that the ring nematode can survive and multiply on blueberries.
Down The Road
New information developed in the last two years indicates that stubby root appears to be a potential nematode issue in the Pacific Northwest, and there are distinct differences in cultivars as to their susceptibility. This indicates that breeding for nematode resistance may be critical to the long-term health of blueberry plantings.
The research into blueberry replant issues will not stop here. Future studies will look at:
• Additional research to further confirm the nematode connection to replant disorder
• Post-plant nematicides
• Solarization studies with plastic
• New pre-plant nematicides
• Rotation crops.