Over the past few weeks, we have been discussing the different types of organisms that can cause plant disease. This includes bacteria, fungi and oomycetes, viruses, and nematodes. Up to this point, we have directed our focus towards bacteria and its diverse affects on plants. This week we shifted gears a little bit to discuss the roles of fungi and oomycetes as plant pathogens.
We first started the week with our lecture on fungi, and how plants react to it. When discussing how plants are attacked, it is important to understand how pathogens are categorized. They are divided into separate classes depending on how they infect the plant. In other words, they are divided into different teams based on how they play the game. There are three main “teams” of pathogens: herbivores, necrotrophs, and biotrophs. For clarity purposes, we will call them the “offense.” Herbivores feed fast, and need to eat 20 times their weight for sustainability. Necrotrophs kill to feed. They eat what is available today, and do not worry about what they will feed off of next. The biotrophs are, in my opinion, the smartest of the bunch. They keep the host plant alive as long as they can so they can continuously feed off of them without having to search for a new host. Biotrophs are better long-term. As with all ballgames, where there is an offensive team, there is also a defensive team. The plant runs its defense differently depending on which team it is playing. It uses different phytohormones to help fight off these pathogens. If a biotroph is attacking the plant, Salicylic acid is the phytohormone that is called on. Likewise, when being attacked by a necrotroph or insect, Jasmonic acid is preferred. These plant hormones help the plant signal the appropriate stress responses to fight off the pathogen.
Just as we studied the different forms a plant pathogen can come in, we also learned about the various ways fungi pass through the physical barriers of the plant to get to the inside.
When a fungus infects a plant, the symptoms are similar to those suffering from drought or starvation. Some of these symptoms include an off-color appearance, wilting of the leaves, and the presence of brownish streaks on the roots. We are scheduled to go back to the gardens on Monday, and although we hope for complete success with healthy crops for the gardeners, I am eager to see if we are able to find any plants that are suffering from a fungal infection. Our trips to the gardens are always fun because we are able to do hands-on activities which helps us get a better grasp on the material we have been studying during lecture.
Above, Dr. Karolina Mukhtar presents a lecture on Oomycetes to our OUTPACE team.
During our lecture on Thursday, we put our focus on oomycetes. My first thought was “I have to know what oomycetes are before I worry about how they affect plants.” To answer the question, broadly speaking, oomycetes are a type of “water mold.” Knowing that it is a type of mold and is similar to fungi made it easier to understand the details of it. First off, although similar to fungi, they are also very different. For example, oomycetes cell walls are made of glucans and cellulose, not chitin like fungi. One defining characteristic is the fact that they can reproduce both sexually and asexually. However, they reproduce asexually most of the time. Oospores are their sexual spores and zoospores are their asexual spores.
In my opinion, one of the most interesting topics of this particular lecture was when Dr. Mukhtar told us a more in depth story on the Irish Potato Blight. I had never heard of the Irish famine before Dr. Mukhtar mentioned it, but it was interesting to see how the disease of one crop could have such dramatic effects on a population. This particular incident killed over 1 million people because a staple food was affected and the people had nothing else to survive by.
We ended the week’s lectures learning about the diseases caused by Phytophthora ramorum, specifically Sudden Oak Death disease. This disease has different symptoms for different hosts. We actually know very little about this disease, so it is a good area to go into for research.
After these lectures and a week away from the gardens, I am anxious to get back next week to see what types of infections we can identify!