When scientists prepare to study something, they do as much research about the topic as possible before designing their study. That way, they will be more successful at asking the right questions and finding the clearest answers. In this study, the scientists studied the organism that causes sudden oak death. They found that the organism is like a fungus. Therefore, they learned everything they could about how fungi spread from place to place. By understanding how fungi spread from place to place, they were able to look for a similar process in the organism that causes sudden oak death.

When homeowners create gardens or do landscaping around their homes, they often use plants that are not native to the area in which they live. These nonnative plants are often transported from country to country, and across the country from nursery to nursery. When a plant is infected with a disease, it is not just the plant that is transported from nursery to nursery. Sometimes, these plants and diseases escape into the natural environment, where they become disruptive to the natural ecosystem. You can see that although we usually think of plants as not being mobile, their reproduction and transport by humans allows them to spread from place to place. If a plant has a disease, the disease can spread as well.

Sudden oak death is a new disease of trees and plants in the United States and Europe. Although scientists are not certain, they believe the fungus-like organism that causes sudden oak death was brought into the United States from another country. At first, the disease was found only in California (figure 1).

Since that time, nursery plants affected with the organism that causes sudden oak death have been shipped to other nurseries across the United States (figure 2).

One way the organism spreads is through the sale of infected nursery plants to homeowners or to nurseries in other locations. When such movement occurs, the organism can be spread quickly across long distances. Spraying plants with a chemical to kill the organism does not always work well enough to keep the organism from reappearing and spreading.

The organism can also spread from plant to plant. When the organism that causes sudden oak death infects a plant, it is hard to stop its spread to other plants and trees. Although the organism damages but does not kill all the tree species that it infects, it does kill some of them. In this study, the scientists wanted to learn how the organism that causes sudden oak death is spread from tree to tree within a forest.

Sudden oak death is a deadly disease affecting four oak species in California and Oregon. It is caused by a fungus-like organism that produces cankers on the bark of oak trees. These cankers look like they are bleeding (figure 3).

The sudden oak death organism also infects 13 other species of trees in California, including California bay laurel, coast redwood, Douglas fir, and bigleaf maple (figures 4a–4d). Although sudden oak death does not kill all the trees that it infects, the infection of any tree adds to the problem. When a tree is infected with the organism that causes sudden oak death, the infection enables the disease to continue to spread.

The scientists thought the organism that causes sudden oak death spreads in a manner similar to fungi (figure 5).

This similarity includes the production of spores (figure 6).

The scientists wanted to learn how the organism’s spores move from tree to tree throughout a forest. The forest they studied contained both evergreen trees and broadleaf trees. The scientists knew that spores of fungi are often transported by wind and water and through soil. Within a forest, sources of water include rain and streams. Soil containing spores can be washed into streams or splashed onto the leaves of new plants.

The scientists studied an area of land covering 166 hectares in California. Within this area of land, 17 percent of the coast live oak and 82 percent of the bay laurel were affected by the organism that causes sudden oak death. Sudden oak death can kill as much as 20 percent of the coast live oaks it infects in a given area (see figure 3 in the “Introduction” section).

Bay laurel trees are not killed by sudden oak death disease, but the disease still causes problems for the tree. The bay laurel tree’s ability to photosynthesize is reduced and more leaves are lost from the tree. This loss is significant because bay laurel trees provide needed habitat and food for wildlife. Even though bay laurel trees are not killed by sudden oak death, the organism produces millions of spores while it is on the leaves. These spores may be spread to oaks, which can be killed by the organism that causes sudden oak death.

The scientists placed 30 traps to catch rainwater across the area of land (figure 7). They placed the traps throughout the area and collected rainwater over 2.5 years. During the rainy season, they collected the rainwater every 10 days. During the summer, when there were few rainstorms, they collected rainwater after each rain. The scientists filtered the rainwater using a very fine filter. This filter was small enough to trap the spores. The scientists performed tests to make sure the spores were the ones that cause sudden oak death. In this way, the scientists were able to estimate how many spores would be released from infected trees during a rainstorm.

To determine how far the spores travel while it is raining, the scientists put buckets in a meadow at the edge of the forest. Bay laurel trees affected by sudden oak death disease were growing at the edge of the meadow. During a rain, the scientists assumed some  spores would travel away from the bay laurel trees, using windblown raindrops as their vehicles. The scientists placed buckets 0 meters, 5 meters, and 10 meters away from the trees in the meadow (figure 8). They collected the rainwater and filtered it in the same way they filtered the rainwater collected from the traps.

The scientists also studied how far spores could travel down a stream. The scientists placed leaves into a screen bag at three locations. They put the bags of leaves in the stream and tied them so they could not float away. If spores were present in the water, the trapped leaves would become infected. The first location was still in the study area. The second location was outside the study area but still in the forest where infected trees were growing. The leaves placed at the third location were 1 kilometer outside the forest (figure 9).

Finally, the scientists wanted to know whether spores could be transported on people’s shoes. For this part of the study, the scientists had the help of students like you. Fifteen students hiked a trail in the study area. They hiked on 3 different days, with five students hiking each day. The scientists repeated this experiment at two different times. The distance the students hiked was 2.4 kilometers. At the end of the students’ hike, the scientists scraped the soil from the students’ shoes into 15 separate bowls. The scientists tested the soil in each bowl for the presence of spores. They then counted the number of students who had transported spores on their shoes.

The scientists found spores in the rain traps placed throughout the forest during the rainy season but not during the summer. At the beginning of the rainy season, the scientists found fewer spores than at the end of the season. The scientists found that the number of spores caught in the traps, however, varied from place to place and from year to year.

The scientists found that the farther away from the forest edge the buckets were placed, the fewer spores were trapped (figure 10).

The scientists found that the leaves floating in the stream within the study area became infected with the sudden oak death organism. During the rainy season, they also found that spores were present in the stream at both other locations, including the location 1 kilometer downstream from the forest. During the dry season, the stream dried out in the forest, and the scientists did not find any spores downstream during that time.

Of the 30 students who hiked 2.4 kilometers, 12 of them picked up spores in the soil on their shoes. Forty-seven percent of the first 15 students’ shoes and 33 percent of the second 15 students’ shoes had spores in the soil that had clung to their shoes.

The findings in this study are similar to research done on similar organisms. In other words, the organism that causes sudden oak death disease is transported from tree to tree in a manner similar to other fungus-like organisms. The presence of this organism is significant because when it is transported to new oak trees, it can cause the death of those trees. The scientists found that this organism is most likely transported from bay laurel trees, where the spores infect the tree but do not kill it.

This study showed how far the organism can travel through rain, streamwater, and soil. In particular, the organism can travel long distances down streams and on the shoes of hikers. Think about the possibility of transmitting the organism that causes sudden oak death across the country from nursery to nursery. If what happened in California could happen in other States, think about what might happen if the organism spread into forests across the United States. Look again at figure 2 in the “Introduction” section. Could sudden oak death get started in your State or in a State near you? The organism that causes sudden oak death is a new invasive species and not much is known about it outside California. Because it can kill oak trees, this is a disease to which we should pay attention.