Phytophthora root and collar rots in fruit orchards in the Czech Republic

Abstract

A recent outbreak of Phytophthora diseases in fruit orchards was identified in the Czech Republic. The diseased trees showed characteristic symptoms including yellowing, wilting and sparse foliage, decreased yields, root and collar rot, and withering and dying of trees. In some orchards up to 10–15, and rarely up to 55%, of trees died. In total, 387 symptomatic trees of nine species from 44 fruit orchards, 16 samples of irrigation water from four orchards and 35 samples of nursery stock, were surveyed in 2012–2018. Oomycetes were recovered from 50.6 % of sampled trees in orchards, from 71.4 % of shipments of ex vitro-produced nursery plants, and from 93.8 % irrigation water samples. Seventeen Phytophthora species and 13 Pythium sensu lato species were recovered. The most frequent species in orchards were Phytophthora cactorum, Phytophthora plurivora and Phytopythium vexans. The most frequent species in nursery stock were P. cactorum, Phy. vexans and Globisporangium intermedium, and Pythium helicoides, Phytophthora lacustris and Pythium litorale were the most frequently found in irrigation water. The most frequent oomycete species recovered from nursery stock were also frequently isolated from orchards and planting material. The pathogenicity of 11 Phytophthora species was assessed in vitro in two frequently used rootstocks – ´St. Julien´ (plum) and ´M26´ (apple). This revealed that the less known and relatively rarely isolated species (P. gregata, P. chlamydospora × amnicola, P. inundata, P. lacustris and P. sansomeana) were more virulent than the more frequently occurring species P. cactorum and P. plurivora, and could pose potential threats to fruit orchards in the future. The increase of Phytophthora in fruit orchards is probably related to the trading and planting of infected nursery stock, to climate change and water stress, and to dismissal of the problem of Phytophthora diseases by stakeholders and inappropriate management. Future investigation should focus on the development of effective disease management, including assessment and selection of rootstocks for resistance.