Phytophthora fragariae

Red stele
Raspberry plants infected with and showing symptoms of Phytophthora root rot.
Common names	Lanarkshire disease
Causal agents	Phytophthora fragariae
Hosts	strawberry, raspberry
EPPO code	PHYTFR

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.

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, they are considered as distinct species.

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 microscopic oospores. 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.