Dahlia Disease Biology: Part 1 – Bacteria vs Viruses

I often wonder if I loved dahlias in high school as much as I do now, and my high school biology teacher taught me about the diseases that attack dahlias as a model to understand basic biology, I probably would have paid a lot more attention. Whether diseases are becoming more common (prevalent) or we are becoming more aware, it seems that most posts we see on social media dahlia groups are questions over the potential disease in our precious tubers and plants. We are embarking on a mini-series (at least 5 parts) to try and better understand the diseases that impact our plants as well as what we need to know to keep our gardens happy. This first part is about general differences between the types of microbes that cause disease. Knowing about the differences between bacteria and viruses will help us understand how our plants are affected.

All organisms are made of cells, but not all life forms. Plants, bacteria and humans are all cellular beings. This means we are able to do certain things like metabolism to keep ourselves alive and reproduction to make future generations. When we worry over fertilizer and watering, we are trying to provide the things plant cells need to metabolize and divide. When the cells divide, the plant grows and eventually blooms. The basic design of the plant cell is a jelly filled center of cytoplasm wrapped in a membrane and outer coating that work together to make a barrier and provide rigidity for the plant. The instructions for everything the plant needs to do is in the DNA nestled inside the nucleus. Plants have two ways to convert energy. During daylight, they use their chloroplasts, but at night they use mitochondria like we have. The color of the leaves and stems are related to the pigments located in the chloroplasts. The mottling we see in diseased plants is partially associated with changes to chloroplasts by nutrient changes or viruses.

Not all cells are made the same. In fact, bacteria are a different type of cell than the types of cells that make plants and humans. All the organelles (cell organs) that you may (or may not) remember from your previous biology classes are parts of the complex cells that make up plants and animals. If you look at the first picture, you’ll see the inside of the cell is very busy. Bacterial cells are much, much simpler with a very basic design.

Bacterial cells have the jelly filling (cytoplasm) inside a membrane. They have a main segment of circular DNA inside and an outer coating that acts as a skeleton. The outer coating is a type of cell wall. Some, but not all bacteria will have little pockets of additional DNA. These pockets of DNA are called plasmids and the bacteria that have them can share them with other related bacteria. This is the equivalent of bacterial sex and can happen between competent species. That word competent is science for a special group of bacteria that can share plasmids.  You  will want to tuck this nugget away for our Part 3 discussion on Crown Gall. Because bacteria can support their own needs, they often live on, or among host cells in the fluids and spaces between the host cells. It’s rare to see a bacteria get inside a host cell, but common for them to get in the spaces between cells.

Another important thing to note is that bacteria are everywhere and are on all surfaces, including you. We often call the bacteria normally found on a host (plant or human) the indigenous bacteria. Most of these are perfectly safe and help keep the host safe. However sometimes there are bacteria that if given the opportunity, will cause an infection. This is another key concept that we will want to be aware of for Part 2 of this mini series on Leafy Gall. The take away idea though is that the world is full of harmless bacteria that are on almost everything and this should challenge our ideas of being “clean.”

Not all diseases are caused by cells. Another microbe exists that is not cellular. These are viruses and are a curiosity to science. Without a cell, they can not reproduce on their own and they can not metabolize. Viruses need a host cell to stage a hostile takeover in which they control the cell and force it to do business for the virus. These are much simpler in anatomy. They have either DNA or RNA, where cells come with both. Viruses package their genetic code in a protein coat called a capsid. That’s pretty much it. DNA or RNA in a protein coat. Where bacteria largely live on and around host cells, viruses have to live inside the host cell.  A plant or human cell being driven to work for a virus can not be a contributing member to the general health of the host. The more cells that are affected, the less likely the plant will be able to support its personal needs.

Diseased plants tend to devote energy to their infections, whether bacterial or viral and that means less energy into growing and reproducing. Studies have found infected plants have stunted growth, struggle to bloom, and in dahlias are poor tuber producers. In light of a plants inability to thrive coupled with a lack of testing ability for home gardeners, the American Dahlia Society recommends that any short-stunted plant, poor producer in blooms or tubers should be assumed diseased and thrown out. You may be thinking that nutrition may be a contributing factor and or plant genetics. A stunted plant may not be diseased, however with the rising issues with disease in dahlias coupled with the investment in our gardens, the risk of disease taking out all your plants vs sacrificing a poor plant to the trash should be considered.

Now that we have a little background to understand the differences between bacteria and viruses, we will be ready to discuss some of the specific diseases that affect dahlias and how they are transmitted between plants. In part 2 of this mini-series, we will start with the very common Leafy gall bacteria.

Cheers and Happy Gardening!