1. Hosts and symptoms

2. Disease cycle

3. Environment

4. Management

5. Importance

6. References

7. External links

Phytophthora fragariae is a fungus-like (oomycete) plant pathogen that causes red stele, otherwise known as Lanarkshire disease, in strawberries and raspberries. Symptoms of red stele can include a red core in the roots, wilting of leaves, reduced flowering, stunting, and bitter fruit. The pathogen is spread via zoospores swimming through water present in the soil, released from sporangia.

Hosts and symptoms

Scientist thought at first that there are 2 varieties of Phytophthora fragariae: P. fragariae var. fragariae, which refers to the pathogen that attacks strawberries, and P. fragariae var. rubi, which refers to the variety that is the raspberry pathogen. Recently, the latter is considered a distinct species, Phytophthora rubi.^[1]

The oomycete does not infect tomato, apple, potato, turnip, cauliflower, chickweed, groundsel, or corn spurry.^[2] In 1958, it was reported that there was a race of P. fragariae var. fragariae that infected loganberries.^[3] Other plants have been reported to be susceptible to P. fragariae, and they include some Potentilla species, rosaceous plants, and Rubus parviflorus.^[4] It is the P. fragariae var. fragariae and P. fragariae var. rubi that attract the most attention and are the most heavily researched.

The Phytophthora pathogens in strawberry and raspberry are similar in many ways morphologically, culturally, and molecularly, but are different in growth.^[5] Nonetheless, they are both variations of the pathogen P. fragariae. Recent research, however, suggests that these two variations may not have been as similar as once perceived. A study done to genetically map these pathogens revealed such a difference in the genome of these two variations, they likely could be considered two completely different species. This research has yet to make any changes to the present day cytology and taxonomy of this pathogen.^[6]

There are different races of P. fragariae, that occur within each form (e.g. var. fragariae, var. rubi) that attack only certain host plant cultivars. Depending on the location, there are anywhere from 5 to 18 different races that have been found to infect strawberries. However, there is not currently an international system to classify the differences of these races.^[4]

The main target of this pathogen is the central vascular cylinder of the roots. The common symptom that gives P. fragariae the nickname of “red core rot” is the red discoloration of these roots. Due to the infection, the stele of the root turns a wine to brick red, and starts to decay and die from the tip upwards. Red color of the roots does not necessarily guarantee the presence of P. fragariae infection. The most dependable way to determine if a plant has P. fragariae is the presence of microscopic oospores.^[4] Another symptom of P. fragariae is badly rooted lateral roots, starting to turn a grey or brown color. Other roots are poorly developed. Root symptoms can be observed starting in late autumn, and young roots are particularly susceptible.

The symptoms on the above-ground parts of the plant generally do not begin to show until late spring and early summer. Overall, the plant ceases to grow, or is stunted in growth, and shows a decline in runner development. The younger plant leaves turn blue-green, and older leaves turn red or yellow. The plant will wilt and collapse. In some cases, not frequently, the plant will merely wilt and die before visible symptoms are able to develop above the ground. The plant may die before fruiting, but if there is fruit produced it will likely be small, deformed, or dry.^[3][4]

The activity of the oomycete ceases when the symptoms are developing in the aerial parts of the plant. This brings about the opportunity for secondary fungi and oomycetes to mask the original Phytophthora pathogen. These secondary organisms have been observed to be species of Pythium and less commonly Rhizoctonia and Phytophthora.^[2]

The symptoms of P. fragariae var. rubi are best seen in the spring. These include small roots, reduction of number of roots, and change in color from white to dark. Removal of the bark will make apparent a reddish discoloration. Smaller roots will rot completely, and the plant itself will wilt. The foliage appears bronzed or a reddish-brown long before the expected autumn timeframe. There are no fresh or new canes produced, and young canes wilt. Some fruiting canes will not break bud or will wilt and dry out. There may be blackish or purple lesions at the base of the plant.^[3][7]

Although not extremely common, the use of PCR to detect this disease is also another diagnostic tool. Specifically, PCR was studied for the detection of Phytophthora fragariae var. rubi. It was found the use of PCR with conventional Phytophthora primers yields more favorable results, is less time consuming, and is more convenient than PCR using specific var. rubi primers. Also, using PCR with primers DC1 and DC5 in the second round was also found to yield favorable results to detect Phytophthora fragariae var. rubi.^[7]

Disease cycle

The infection can be spread from field to field by unsanitized tools or equipment, floodwater, or any other activity that will move infected soil. Also, the replanting of infected nursery stock will spread the infection of P. fragariae.^[4]

Environment

There are three main components of the environment that affect the effectiveness, speed, and ability of P. fragariae to infect: soil moisture, temperature, and pH. P. fragariae occurs frequently in wet, cool soils. Soil moisture has been proven to carry the greatest importance. Soils that receive lots of steady water from rain or melting snow, as well as soils with poor water drainage that become waterlogged, produce the ideal conditions. In fact, there is a direct correlation between the soil moisture and the percentage of roots that were infected with P. fragariae. Temperature plays an important role in the environment needed for P. fragariae. The disease will grow in the temperature range from {{convert|5-25|°C}}. However, as you approach the lower and higher ends of the temperature range, the disease develops and spreads more slowly. The optimum temperature range for P. fragariae is between {{convert|10-15|°C}}. The effect of pH is more difficult to understand; oospores germinate best in soils with a higher pH. In contrast, soils with a lower pH are better suited for the mycelium and more mature parts of the oomycete.^[4]

Management

The main form of spreading of P. fragariae is through infected material. Strict regulations in many countries have been instituted regarding the transportation and disposal. For instance, Norway requires an official statement saying that the location you are planting has not been known to host P. fragariae and that the plants you are planting show no symptoms.^[9] Fosetyl-aluminum and fungicides have been approved for use against P. fragariae. These chemicals, when applied in late autumn just before the infection of new roots, have been proven to be effective against var. fragariae, but not var. rubi. Fungicides containing phenylamides have been approved when applied in early spring or late autumn. Some strains with phenylamide resistance have been found and have proven to be problematic in Germany and North America. Resistant plants have been bred and are commercially available. The resistance may be race specific and usually used where one strain is predominant. Due to the importance of water in germination and infection, good drainage can help reduce the proliferation of the disease.^[3]

Importance

References

External links

• Index Fungorum
• [https://www.webcitation.org/5QK694Uju?url=http://nt.ars-grin.gov/fungaldatabases USDA ARS Fungal Database]

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