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Plantwise Technical Factsheet

root rot (Phytophthora megasperma)

Host plants / species affected
Actinidia deliciosa (kiwifruit)
Aesculus hippocastanum (horse chestnut)
Alcea rosea (Hollyhock)
Asparagus officinalis (asparagus)
Brassica oleracea (cabbages, cauliflowers)
Brassica oleracea var. botrytis (cauliflower)
Brassica oleracea var. capitata (cabbage)
Brassica oleracea var. gemmifera (Brussels sprouts)
Brassica rapa subsp. pekinensis
Buddleia (Butterflybush)
Carthamus tinctorius (safflower)
Castanea sativa (chestnut)
Celtis australis (European nettle wood)
Chamaecyparis lawsoniana (Port Orford cedar)
Cicer arietinum (chickpea)
Cytisus (Broom)
Daucus carota (carrot)
Dianthus caryophyllus (carnation)
Foeniculum vulgare (fennel)
Juglans regia (walnut)
Malus domestica (apple)
Matthiola incana (stock)
Medicago sativa (lucerne)
Morus alba (mora)
Narcissus (daffodil)
Parthenium argentatum (Guayule)
Pinus (pines)
Prunus armeniaca (apricot)
Prunus avium (sweet cherry)
Prunus cerasus (sour cherry)
Prunus domestica (plum)
Prunus dulcis (almond)
Prunus mahaleb (mahaleb cherry)
Prunus persica (peach)
Prunus salicina (Japanese plum)
Pseudotsuga menziesii (Douglas-fir)
Ribes uva-crispa (gooseberry)
Rubus idaeus (raspberry)
Saccharum officinarum (sugarcane)
Solanum lycopersicum (tomato)
Solanum tuberosum (potato)
Sorbus aria (whitebeam)
Sorbus aucuparia (mountain ash)
Vicia faba (faba bean)
List of symptoms/signs
Fruit  -  lesions: black or brown
Fruit  -  lesions: on pods
Leaves  -  abnormal colours
Leaves  -  abnormal leaf fall
Leaves  -  necrotic areas
Leaves  -  wilting
Leaves  -  yellowed or dead
Roots  -  cortex with lesions
Roots  -  necrotic streaks or lesions
Roots  -  reduced root system
Roots  -  rot of wood
Roots  -  soft rot of cortex
Stems  -  canker on woody stem
Stems  -  dieback
Stems  -  discoloration
Stems  -  discoloration of bark
Stems  -  gummosis or resinosis
Stems  -  necrosis
Stems  -  rot
Stems  -  wilt
Vegetative organs  -  internal rotting or discoloration
Vegetative organs  -  soft rot
Vegetative organs  -  surface lesions or discoloration
Whole plant  -  damping off
Whole plant  -  discoloration
Whole plant  -  plant dead; dieback
Whole plant  -  seedling blight
Whole plant  -  wilt
P. megasperma is primarily a root-rotting organism, causing most serious losses on apples, stone fruits and brassicas.

In apples growing in New Zealand, Robertson and Dance (1971) attributed the following symptoms to P. megasperma: delayed bud break, stunted growth, orange discoloration of the bark, and crown cankers. These cankers extended from the crown of the root system to ground level and along the lateral roots. In a lot of cases, the canker girdled the trunk, and the bark within the canker was completely rotted and the outer wood discoloured. In pathogenicity tests, P. megasperma was found to invade apple tissue more slowly than P. cactorum. P. megasperma was isolated more frequently than P. cactorum, however, from the sites surveyed and was unequivocally established as a serious pathogen of apple.

Mircetich and Matheron (1976) were able to isolate P. megasperma, P. cambivora and P. drechsleri all from the same diseased cherry trees. In subsequent pathogenicity tests, P. megasperma and P. cambivora both caused severe root and crown rot, whereas P. drechsleri only caused rotting of feeder roots. In field-infected cherry trees, major symptoms were extensive cankers extending up the trunk, and root rot.

Root rot of Brassica species caused by P. megasperma was first described by Tompkins et al. (1936) in the USA, and has since been observed in several other countries, where it is regarded as a significant disease. In cauliflower and Brussels sprouts, root infection is followed by leaves turning red to purple from the margins inwards, wilting and death. Plants at all growth stages are susceptible. The tap root may become rotted through, showing a dark necrotic appearance. In earlier stages of the syndrome, the cortical tissue pulls away readily from the stub. Adventitious roots may form above the rotted portion.

In the majority of other recorded hosts, only relatively minor symptoms such as rotting of feeder roots are observed, with the exception of asparagus, in which serious losses are caused by spear rot (Falloon et al., 1983).
Prevention and control
Host-Plant Resistance

In Malus and Prunus species, the use of resistant rootstocks in nurseries has been investigated. Broadbent et al. (1996) tested a range of peach (Prunus persica) cultivars as seedlings for resistance to a range of Phytophthora species, including P. megasperma. P. megasperma was not as aggressive as P. cinnamomi or P. cambivora, but all rootstocks tested were susceptible. They also obtained similar results for myrobalan plum (Prunus cerasifera) seedlings.

Wilcox and Mircetich (1985a) found that "Mazzard" (Prunus avium) cherry rootstock was more resistant to P. megasperma than "Mahaleb" (Prunus mahaleb) in California, USA.

Cultural Control and Sanitary Methods

Soil water management techniques, particularly those that minimize prolonged periods of flooding (Wilcox and Mircetich, 1985b), are regarded as one of the most effective ways of managing all diseases caused by P. megasperma.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

P. megasperma is considered a major pathogen of apples in New York State, USA (Jeffers et al., 1982) and in New Zealand (Robertson and Dance, 1971) and Prunus species in California, USA, particularly cherry (Mircetich and Matheron, 1976; Wilcox and Mircetich, 1985a). Generally, it is considered a less aggressive pathogen of apple than P. cactorum (Erwin and Ribeiro, 1996), but the most serious Phytophthora pathogen of cherry (Wilcox and Mircetich, 1985a).

The fungus can also have a major economic impact on Brassica species, particularly cauliflower, in the USA (Hamm and Koepsell, 1984) and in the UK (Geeson et al., 1990). The disease was more severe in autumn-planted cauliflower compared with spring plantings in Oregon, USA, presumably due to longer period of cool and wet soils during the growing season (Hamm and Koepsell, 1984).

Generally, P. megasperma is one of the less aggressive species of Phytophthora, and causes debilitation rather than substantial plant death.
Related treatment support
Plantwise Factsheets for Farmers
Valencia Legua, A.; CABI, 2012, English language
Valencia Legua, A.; CABI, 2012, Spanish language
Khalid, L.; CABI, 2014, English language
Cai, L.; CABI, 2014, Chinese language
Okee, J.; CABI, 2012, English language
Pest Management Decision Guides
Khan, Y. S.; CABI, 2013, English language
Khan, Y. S.; CABI, 2013, Urdu language
Moses, E.; Akrofi, S.; Beseh, P.; CABI, 2016, English language
Mbugua, T. W.; CABI, 2017, English language
Somachandra, K. P.; Perera, M. S. K. K.; Kathuriarachchi, R.; Herath, Y. M. C. K.; CABI, 2015, English language
External factsheets
Ohio State University Extension Fact Sheets, The Ohio State University Extension, 2008, English language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, English language
Kentucky IPM Scout Info Factsheets, University of Kentucky, 2005, English language
University of California IPM Pest Management Guidelines, University of California, 2008, English language
Crop Science Extension & Outreach Factsheets, College of ACES, University of Illinois, Urbana Champaign, USA, 2015, English language
Video factsheets
Agropedia ICRISAT PPT-Videos, IIT, Kanpur, 2014, English language
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