US (CA): Potential impact of El Niño on Phytophthora diseases

El Niño conditions in the Monterey Bay area this winter are predicted to bring above average rainfall, which can come in drenching rains that flood and waterlog soils for extended periods. At the same time, winter air temperatures are predicted to be warmer than normal and consequently soil temperatures will be too. So, what impact might this have on plant diseases? At least one important group of plant pathogens might be favored by these conditions. Phytophthora are fungus-like pathogens that develop particularly well in wet conditions and some species, such as P. cinnamomi, do best in moderately warm conditions. They all need water to reproduce, spread and infect plants. During rain downpours, infested soil or spores can move readily in water runoff and splashing water. Waterlogged soil predispose nursery plants to root and root-crown infection caused by Phytophthora cactorum, P. tentaculata and other soil-borne Phytophthora species. The longer leaf wetness periods will favor the production of aerial spores and infection of P. ramorum (causing sudden oak death) and P. infestans (causing potato and tomato late blight), as well as other species that mostly attack and develop in aboveground portions of the plant.

Many Phytophthora species cause destructive crown and root rot diseases of herbaceous and woody plants in landscapes and nurseries. The leaves of plants affected by Phytophthora root or crown rot can first appear drought stressed. Leaves may turn dull green, yellow, brown, or in some cases, red or purplish (fig. 1).

Aboveground symptoms of Phytophthora root rot can include wilting, yellowing and browning of foliage. Root rot symptoms of this fir tree are shown in fig 3. Photo: S. A. Tjosvold.

Plants often wilt and die rapidly with the first warm weather of the season. Phytophthora species can infect small and large roots, turning them brown and soft, and rendering them useless to the plant. Roots at the bottom of the nursery pot are frequently first infected. Infection of the bases of stems or root crowns is possible when transplants are planted too deeply. Often root and crown rot diseases are first noticed in the most susceptible plant species, in the most poorly drained area of the nursery, or in pots that have poor quality soil. Some Phytophthora species, such as P. ramorum, P. infestans and P. nicotianae, cause various plant blights. Although hosts of P. ramorum show a range of symptoms, in general the disease is characterized by irregular necrotic leaf lesions, rather than distinct leaf spots. Leaf infections can develop down the petiole and into twigs. Often, such as in camellia, infected leaves fall off before the lesion reaches the petiole. Infections may occur initially on stems or move into stems and cause blights in which stems and associated leaves wilt, become necrotic and die. A distinct dark line can mark the advance of the infection on some species.

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