When stars make an appearance in Hollywood the paparazzi are sure to follow, cameras in tow. In Santa Cruz it seems that is not the case, not unless you count the handycam my fellow blogger, Janice Alexander, set-up. Though we had the world’s best and brightest the only fanfare was a round of applause after the celebrities gave their presentations. Who were these cynosures you ask? World leaders in the field of Phytophthora.
In the coming weeks, I’ll be working to digitize, edit and post the talks from the Fourth Sudden Oak Death Science Symposium, but until that work is complete, I’d like to highlight a few of the informative and inspirational presentations.
The first-up on my Top 10 list is a talk given by Dr. Jennifer Parke, plant pathologist at Oregon State University.
Dr. Parke presented her lab’s work, done in collaboration with Dr. Nik Grünwald’s lab at USDA-ARS Corvallis, on a systems approach to detecting sources of Phytophthora contamination in nurseries. The system, modeled after the food industries Hazard Analysis Critical Control Points (HACCP) which ensures food products are not contaminated during processing, focuses on Critical Control Points (CCPs) that is points at which a serious hazard of Phytophthora contamination can be controlled.
Over the last 3 years, Parke and Grünwald have worked with four nurseries on identifying the CCPs in nursery production and developing best management practices to mitigate contamination. Though the study focuses on Phytophthora, a closer look at the best management practices quickly reveals its application to controlling other pathogens.
The icing on the cake? The researchers have designed a free online course for nursery growers to help them identify and learn to manage Phytophthora diseases in their operations.
If my last blog was not enough to make you think Phytopthora are incredible, perhaps today’s post will convince you.
In talking with Steven Swain, our Horticultural Advisor about Phytopthora he shared what he what he called, “The coolest things about Phytophthora: the zoospores.” Here are the highlights, with some additions from yours truly.
For starters the zoospores accomplish movement in a unique way. If you are familiar with single celled organisms , you’ll know that many of them “swim” by use of cilia or flagella that often stick out the back and accomplish movement by a series of twists and turns. Phytophthora zoospores actually have two flagella: a whiplash flagellum that sticks out the back and pushes the zoospore along and a tinsel flagellum that protrudes out the front. The tinsel flagellum has protrusions like branches on a tree, that open and close, thereby pulling the zoospore along.
Tinsel flagellum of Phytophthora infestans source unkown
The flagellum also doubles as an “antennae”, so to speak, guiding the zoospore through the world. It uses different taxes, pronounced tæksi?z, to accomplish this feat. Taxis is simply (or not so simply), an innate behavioural reponse to a directional stimulus. Phytophthora flagella exhibit several: phototaxis (attraction to light),chemotaxis (response to chemical gradients), geotaxis (attraction due to gravity), and electrotaxis (response to a electrical field).
What all this means is, the zoospores can detect light and swim toward or away from it, they can sense chemical gradients and react accordingly, they can detect gravity and swim up, and they can detect unique electrical fields. In the case of P. sojae, it swims along through the soil until it senses the distinct electrical field of soy plants. Once detected it makes its move. Any other plant's field will repel the zoospores.
This single celled organism is capable of experiencing the world in so many dimentions it is incredible. Amazing zoospores!
Micrograph of zoospores (courtesy of wikimedia)
Education and outreach on sudden oak death often address the beastly aspects of Phytophthora ramorum - namely the mortality it causes and the long term impacts. This is to be expected with an introduced pathogen as its effects can be devastating.
There is another side of course - the beauty of this beast. While looking at micrographs of P. ramorum sporangia, I was reminded of the beauty and complexity of this oomycete.
Micrograph of sporangia courtesy of Dave Rizzo
Unlike other Phytophthora species that you may be familiar with - which infest roots and are primarily soil and water borne - it is thought that P. ramorum mainly spreads aerially and tends to infest the above soil parts of plants. It does this by producing sporangia (pictured above) which contain zoospores. The sporangia themselves can be airborne, and when they land on a suitable host, they either germinate directly or release their bounty of tiny zoospores (8-10 on average). When conditions are right (wet and/or optimally 18 -20°C) the zoospores then readily infect the host.
According to work done by the Garbelloto Lab, it may take as many as 10, 000 sporangia to infect an oak! This seems astounding, until you consider that most of these sporangia are formed on nearby foliar hosts, like California bay laurel (Umbellularia californica), where a small amount of infected tissue can contain thousands of these spore producing bodies.If you are a susceptible oak within 10m of a bay tree, consider yourself in trouble. In a strong, wind-rain event, sporangia can travel up to 1/2 a mile!
P. ramorum's biology is fascinating and complex and though it may be easier to focus on the beastly qualities of this oomycete, every now and then, I like to pause and admire its beauty.
Nicole’s last blog entry about tomato late blight, caused by Phytophthora infestans, was pretty interesting, and it inspired me to look up some more information about the disease and its management. Given that both tomato late blight and sudden oak death are caused by members of the Phytopthora genus, one might expect some common themes to show up. And they do. Following are what I think are some of the more interesting parallels, with quotes about late blight (in italics) from the University of California Integrated Pest Management Program:
“Late blight is found when humid conditions coincide with mild temperatures for prolonged periods. When humidity is above 90% and the average temperature is in the range of 60° to 78°F, infection occurs in about 10 hours.” As I noted in an earlier blog post, researchers at UC Davis have observed that the greatest quantities of P. ramorum spores are detected in the field when we have rains during April, May, and June. (Incidentally, these conditions can also promote tree infection by a variety of other fungi.)
“The fungus overwinters in potatoes, tomatoes, hairy nightshade, and possibly in the soil. Spores of the fungus are easily spread by wind to other plants . . . . Remove any nearby volunteer tomato and potato plants and nightshades . . . . Disc tomato fields in fall to eliminate a winter reservoir for the fungus.” The resemblance between late blight and the diseases caused by P. ramorum is pretty striking here. Whereas P. infestans overwinters in the members of the nightshade family mentioned here, P. ramorum appears to overwinter—and, perhaps more importantly, oversummer—mostly in leaves of the California bay laurel. Because of this, experiments on controlling P. ramorum at a larger scale in the field have primarily involved removing the hosts that play the largest role in harboring and transmitting the pathogen (bay laurel and tanoak). Note that this doesn’t necessarily entail the removal of all of these hosts in a given area—rather, strategic removal of the ones near tanoaks or true oaks, within the distance that spores can be expected to fly in rainy weather, is warranted.
P. ramorum survival structures (chlamydospores) in bay laurel leaf (chlamydospores are blue). Photo courtesy of Jennifer Parke, Oregon State University
“Apply a protectant fungicide before disease development begins; once an outbreak occurs in a field, it is important to apply additional applications at regular intervals. Coverage must be thorough for applications to be effective.” In both the nursery and forest situations, fungicides are also primarily useful as prophylactics, rather than cures, for P. ramorum infection. When applied to nursery plants that are already infected with P. ramorum, most of the common contact fungicides (such as mefenoxam or metalaxyl) will suppress symptoms but not eliminate the pathogen from the plant. Similarly, the systemic fungicide phosphonate, which is useful as a preventative, is usually ineffective after oak or tanoak trees are infected with P. ramorum. Only in cases of extremely small infections on coast live oaks can phosphonate sometimes slow down the course of infection.
Doug Schmidt from UC Berkeley injecting tanoak tree with phosphonate fungicide. Photo courtesy of Radek Glebocki, UCCE
“Tomato varieties resistant to certain races of the late blight fungus are grown where the disease occurs regularly.” Nicole mentioned the availability of these resistant varieties in the case of tomatoes. The concept of growing resistant trees is one that has also occurred to people who research P. ramorum. However, since our understanding of this particular Phytophthora is still young, there is a long way to go. Because of the multiple steps required to identify the genetics behind resistance and to breed and test seedlings for resistance both in the greenhouse and in the field—complicated by the longevity of trees as compared to field crops—the identification of tanoak and oak individuals that are resistant to P. ramorum could easily be 15-30 years away.
Not that this is meant to strike a note of helplessness or doom. Rather, it just goes to show that versatility, toughness, and adaptability seem to be traits that many species of Phytopthora share—imperiling tomatoes and tanoaks alike!
P. infestans infecting a potato sprout touching an already-infected tuber. Photo courtesy of D. Inglis
