A Citrus Problem [Finally the Answer!]

A few weeks ago at work, an inspector brought in some citrus leaves from a nursery. He had gone there to survey citrus trees for pests and diseases and he found this:

Symptoms on the upper surface of the leaves

The inspector said that all of the citrus trees had this problem. Both he and the nursery owner were wondering what was causing these symptoms. It's my job to figure out stuff like this, so I took the samples and logged them in to our specimen tracking system.

So, what do you think is the cause of these symptoms?

Sorry I don't have a picture posted of the underside of the leaves. I tried, but blogger kept inverting the picture. The yellow spots on the upper surface of the leaves are also visible on the underside of the leaves.

Feel free to post your ideas in the comments. On Wednesday 9/15/10, sometime next week, I'll update this post with the diagnosis that I gave to the inspector and owner. Sorry about the delay, but I have a deadline that I have to meet.

Okay, I'll give you a hint. Ask yourself whether the cause is biotic or abiotic.


THE ANSWER

I appreciate your patience in waiting for the answer to the citrus problem. I have been buried under job applications, but decided to come up for air...

So, because I was the only one around when the inspector arrived with the citrus sample, it fell upon me to come up with the diagnosis for the symptoms on the leaves. I'm going to over the thought process that I went through to arrive at the correct answer.

I looked at the leaves and observed the yellow (chlorotic) spots. The first thing I asked myself was: is the cause biotic (a live organism) or abiotic (nutritional, injury, environmental, genetic, etc.)? If the cause was biotic, I would expect for there to be some necrosis (brown or black plant tissue) mixed in with the yellow spots. There was no necrosis on the leaves. The next thing that I looked at was the distribution of the spots. You'll notice that there are no yellow spots around the base of the leaf near the stem. Also notice in the picture above, that in the leaf on the right the spots are distributed mainly on one side left of the midrib). So the distribution of symptoms are not random. Rather, they are clustered along the top to middle and along one side of the leaves. If the cause were a live organism (plant pathogen), the chlorotic spots would be distributed on the leaves in a random pattern. So, because no necrosis was present and because the spots were not random, I determined that the cause was abiotic.

I asked myself what caused the spots on these leaves? If you'll notice, the spots look kind of like droplets; like maybe spray drops. So, was something sprayed on the citrus trees? Yes, I do believe something was sprayed on the citrus trees in the nursery. But, what? A chemical spray? The most likely chemical spray would be herbicide. But, why would a nursery manager or staff spray herbicide on their valuable and expensive citrus crop? I have seen herbicide damage on plants before and this looked like what I would expect, so I decided that the cause had to be herbicide. Sometimes unintentional application of herbicide can kill a crop depending on the amount applied, but clearly in this case, the amount applied did not kill the trees. The damage was enough to cause phytotoxicity injury to the citrus, but not kill them. So it was a mistake.

So, how could a mistake like this happen? Think it through. The purpose of using herbicides in the first place is to kill weeds. The trees were in a greenhouse nursery; enclosed in a building. We can safely assume that herbicide was not sprayed inside the greenhouse. So, it was sprayed outside of the greenhouse. But that doesn't explain how the herbicide got on the citrus trees. Think about the equipment used to spray chemicals. Ah, the sprayer. They used a sprayer to spray herbicide on the outside of the greenhouse. Then they used the same sprayer to spray something else inside the greenhouse.

What did they spray inside the greenhouse? Take another look at the yellow spots on the leaves. They look kind of big; and there are a lot of droplets that look clumped together. Usually herbicide spray produces a rather fine spray, with a small diameter drop size. These drops look big, with a large diameter droplet size. So the damage looks like herbicide injury, but the droplet size looks like something else. What? What else can be sprayed that has a larger droplet size than herbicide spray? Well, of course...fertilizer spray! Do you see now what happened?

My diagnosis: herbicide phytotoxicity.

My analysis (I had to be tactful about what I wrote): The most likely cause is herbicide injury due to herbicide residue on the sprayer used to apply fertilizer to the citrus trees.

I sent the final report to the inspector. A few days later, he came to see me. He said he got very curious about the whole thing when he received my report. So, he went back to the nursery to try to find out whether that was really what happened. He walked around the outside of the greenhouse and noticed that there were dead weeds all along the outside perimeter of the greenhouse. Then, he went to talk with the nursery manager. The manager admitted that they had used the same sprayer to fertilize inside the greenhouse that they had used to spray herbicides outside. He said the nursery staff forgot to wash out the sprayer before applying fertilizer to the citrus.