Phytophthora Fragariae - Hosts and Symptoms

Hosts and Symptoms

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. The fungus does not infect tomato, apple, potato, turnip, cauliflower, chickweed, groundsel, or corn spurry. In 1958, it was reported that there was a race of P. fragariae var. fragariae that infected loganberries. Other plants have been reported to be susceptible to P. fragariae, and they include some Potentialla species, rosaceous plants, and Rubus parviflorus. 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. 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.

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.

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 microscropic oospores. Another symptom of P. fragariae is badly rooted lateral roots, starting to turn a grey or brown color. Other 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 will 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.

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

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.

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 DC1 and DC5 in the second round was also found to yield favorable results to detect Phytophthora fragariae var. rubi.