WORKING DRAFT VERSION
In the first post in this series last week, I reported on some of our management success in controlling dieback in Early Pride citrus. I took a look at the symptoms of the problem and examined the progress of the disease over the course of a year in a new block we planted. Although I didn’t go into detail last week, I made the point that most of the dieback we saw was due to some sort of physical trauma. This week, I’ll take a detailed close-up look at those symptoms, and illustrate how I came to the trauma conclusion. In my next report due later this week, I’ll go over the findings of the USDA regarding secondary pathogens, and the treatment programs we used to help contain the problem in our grove.
I initially planned on two reports describing our Early Pride problem, but the extensive photography in my post this week is going to result in having to spread my Early Pride analysis out over three reports. To summarize last week’s report, I talked about tree condition, and showed several examples from the dieback from about a year ago and compared that to imagery of trees in the same section of the growth taken last week. The difference is dramatic.
The main message from last week was that I presented a list of general practices that can be used to minimize the kind of dieback we saw in our grove. Among the most important of these practices is ensuring that you don’t plant trees that are too small. Another important consideration is to minimize physical injuries.
Before going into the detail, I’d like to give you a general overview of the symptoms with a little closer view than I showed last week. The gallery below shows eight such examples of dieback representative of what we saw. What’s interesting is that the necrotic areas in the twigs seem to be clearly delineated from the healthy areas. This gives us an advantage in pruning, because it makes the area to prune easy to identify. Sometimes the entire branch would be chlorotic or necrotic, while in other cases, the necrotic areas would alternate with the healthy areas. Both these extremes as well as intermediate examples are shown in the gallery below.
• Credit:Steve Rogers• Camera:k• Taken:November 18, 2013• Exposure bias:+1000000/5310704EV• Focal length:0.806451612903mm• ISO:100• Shutter speed:1/0s
Close Up Analysis and Discussion of Causes
As outlined in our previous post, we conducted several surveys to document the distribution of dieback in this grove. Basically, we were able to attribute dieback to the general causes and percentages shown in the graph below. We were unable to attribute dieback symptoms to a specific cause in over 50% of the trees we examined. As discussed below, this might have been due to the possibility that the injury was not visible or it might represent a manifestation of some kind of physiological disorder. In about 18% of the cases, we noted dieback was associated with a suspected physical injury of some type. In these instances, injury was found near the dieback, but it may not have been clear at the time whether or not the injury appeared before or after the dieback. In about 4% to 5% of the cases we looked at, dieback and tree attrition could be traced back to some kind of girdling event.
Girdling from Tapes, Ties and Staples
Overall, girdling seems to have accounted for about 3% to 4% of the dieback we saw in our grove. It appears that Early Pride is sensitive to scraping injuries such as what you get when little tree scrape against a tape or a tie. Further, it appears that Early Pride, unlike other citrus trees, is unable in some cases to force ties outward as they grow, resulting in the trunk being girdled. I don’t believe it would have been possible to know about this unusual type of injury in Early Pride as compared to other citrus trees, because there are so many different factors that go into raising young trees.
Perhaps this is due to trees being too small when they were secured with the ties. But I don’t recall other citrus varieties showing this extent of injury from tapes and ties before. Girdling obviously prevents the movement of nutrients and water up and down the tree, and thus the stem, branch, and leaves the distal to the injury will die. The gallery below shows eight such examples of injuries and died back that we could trace directly to injuries from tapes ties and Staples.
Irritation from Planting Stakes
One interesting manifestation of disease in the Early Pride dieback we saw was a kind of irritation that seemed to result from planting stakes. In the examples shown below, I have illustrated injuries to the trunk at the point of contact between the metal stake and the tree. It looks like there may have been some kind of puncturing or scraping of the trunk that occurred during handling or transport of the trees. This resulted in a wound, which looks like it became infected, and this ultimately led to to dieback in the upper part of the trees on the stems and leaves. This irritation was not limited to metal stakes. We also noticed it in a number of instances that involve bamboo stakes as well.
Miscellaneous Physical Injuries
In addition to the injuries outlined above, there were a number of other types of physical trauma. These types of injury are illustrated in the floor gallery images below. As you can see, the bark and the truck seems to have sustained some kind of splitting or wound, and we could trace die back in these trees directly back to these injuries that we illustrated here. It’s not clear how these injuries developed, but we speculate that they might have been caused by rough handling when we transported or planted the trees, when they were batted about by the wind at some point, or some such similar cause. Because we can’t specifically identify the nature of these worlds, is difficult to come up with a specific practice to help minimize them.
Unidentified Causes of Dieback
Finally, a reasonable percentage of diabetic we saw was difficult to attribute to any specific visible cause. These trees may have sustained by back due to some kind of unnoticeable injury, which subsequently became infected by secondary pathogens, leading eventually to die back. We inspected such trees closely, but it was difficult to see a particular wound or point of entry where this would have occurred. However, at this point, based upon the above overwhelming evidence that physical trauma is a primary cause of the type of diabetic we saw in our early pride trees, we have to think that even the trees and identify causes of diabetic were due to some kind of physical injury we could not see.