Mottles

Mottles are symptoms of viral disease. Mottles are irregularly colored, spotted, or blotched areas. These discolored areas are usually yellow. Simple, right? Well, those words also describe mosaic diseases, which are different from mottles. I think.

Tobacco vein mottling virus (RJ Reynolds) CC BY-SA 3.0

---

Note: Have you ever started something, thinking it would be simple, and come to find out that there is a lot more to it than you ever would have thought? Mottles are like that.

---

Viral families

There are several families of viruses that cause mottle diseases in plants. They cause mosaic diseases, too. And each of these families causes dozens, maybe hundreds, of diseases. I had no idea. There are even sub-viral agents, which lack the genetic instructions needed for replication. Because of this, they use helper viruses to set up household in plant victims. I swear, the more I learn, the stranger the world gets.


Mottle diseases

There are countless plant diseases that present with mottles, including:

• bean pod mottle (Secoviridae) - spread by bean leaf beetles to legumes
• blueberry leaf mottle virus (S.) - spread by infected seeds to blueberries and grapes
• broad bean mottle virus (Bromoviridae) - spread by infected sap to legumes
• cowpea mottle virus (Tombusviridae) - spread by beetles and infected seed to cowpeas and other  legumes and many members of the nightshade family
• cucumber green mottle mosaic virus (Tobamovirus) - spread by infected sap to cucurbits
• pepper mild mottle virus (Tobamovirus) - spread by mechanical transmission and infected seeds to bell peppers and chili peppers
• soybean yellow mottle mosaic virus (Tombusviridae) - spread by infected sap to soybeans
• strawberry mottle virus (S.) - spread by aphids to strawberries
• tobacco vein mottle virus (Potyviridae) - spread by aphids to tobacco, groundcherries, and other nightshade family plants.

​
Mottling can also occur in response to cold temperatures. Or it may indicate a zinc deficiency. Air pollution can also cause mottling. In the case of mottle-leaf in citrus, it really gets confusing. The jury is still out on whether this condition is caused by too much nitrogen, not enough nitrogen, mineral deficiencies or toxicities, or insufficient soil organic matter (SOM). Yup. No one knows. Confused? Me, too.


Mottle or mosaic?

The distinction between mottling and mosaic patterns must be subtle, because I have had a very difficult time finding any. But it might be important in diagnosing plant disease. I’m not sure.


What I have learned is that mosaic patterns tend to be dark green on green, yellow on green, or gold on green, with either distinct or fuzzy borders, and in systematic patterns. Mosaic diseases may also appear with vein banding, stripes, streaks, and variegation.


Mottles, on the other hand… well, um, well, mottles can look exactly like mosaic diseases, but without the variegation, banding, or stripes. Rats! The only information I could find was that mottles can also appear on fruit, while mosaics tend to be seen only on leaves. I think. And then there are diseases like cucumber green mottle mosaic and soybean yellow mottle mosaic virus. Agh!!!

Apparently, even the experts haven't sorted all this out yet, so don't feel as though you should understand it all. I certainly don't!


​Maybe mottles and mosaics are the same thing after all. If you know the answer, please share it with the rest of us in the comments. 


Bottom line, if you see uncharacteristic coloration on leaves or fruit, you need to take a closer look and consider what might be causing it before a full blown infection wipes out your crop.