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In 2019 and 2020, wilting, withering, and root and basal stem rot of potted herb plants
Rosmarinus officinalis and Thymus vulgaris ‘Faustini’, and leaf spots of Rhododendron
‘Cunningham’s White’ were observed in three localities (private greenery, gardening center,
and ornamental nursery) in Prague, western and central Bohemia. The species Phytophthora
nicotianae Breda de Haan was consistently isolated from infected tissues on PARPNH (Jung et
al. 1996) medium. Isolates formed fluffy to slightly stellate colonies on 20% V8 agar (V8A)
plates at 25°C after 5 days, but on carrot agar plates colonies were uniform with no aerial
mycelium. The cardinal growth temperatures were: min. 8°C, optimum 27 to 32°C, and max.
37°C on V8A. Radial growth (three isolates) was 7.1 to 7.8 mm/day at 25°C. The isolates were
usually heterothallic, but the isolate from Rosmarinus sometimes in old cultures produced
smooth-walled, spherical oogonia ranging from 18.7 to 26.6 μm (n = 40) in diameter. Oospores
(14.5 to 22.8 μm in diameter, n = 40) were aplerotic with a 0.9- to 1.5-μm-thick wall.
Amphigynous antheridia averaged 11.8 × 11.7 μm (height × width, n = 40). Noncaducous
papillate (sometimes two papillae occurred) sporangia varied from ellipsoid, ovoid, pyriform, to
globose in shape and were 34.3 to 78.6 × 23.0 to 50.1 μm (n = 60). Terminal chlamydospores
(n = 60) were 13.5 to 57.5 μm with a 0.7- to 1.5-μm-thick wall. Morphological characteristics
resembled those described for P. nicotianae (Erwin and Ribeiro 1996). The isolates were
deposited in the Czech Collection of Phytopathogenic Oomycetes of RILOG Pruhonice under
nos. 1087.19, 1144.20, and 1101.19. These three isolates were sequenced for the rDNA ITS
region, partial NADH1, and partial COX1 gene. Obtained sequences were deposited in GenBank
(accession nos. MW762935 to MW762937 and MW762610 to MW762615) and compared with
sequences in the GenBank database using BLAST. The ITS sequences showed 99.7 and 100%
similarity to reference P. nicotianae sequences (KJ494902 and MH219888). The sequence of
isolate 1087.19 showed a 1-bp difference, while isolate 1144.20 contained ambiguous bases. The
COX1 and NADH1 sequences of isolates 1087.19 and 1144.20 were identical to reference P.
nicotianae sequences (accession nos. MH477752 for COX1 and AY564023 for NADH1). The
sequences of isolate 1101.19 differed in two positions in both genes and showed 99.7%
similarity to the same reference P. nicotianae sequences. The three isolates were tested for
pathogenicity on all three host species (10 plants per isolate per host species). Two-year-old R.
officinalis and T. vulgaris plants were inoculated with three 5-mm V8A plugs with mycelium
from 7-day-old colonies by inserting into the substrate near the collar. Leaves of 3-year-old
Rhododendron ‘Cunningham’s White’ were inoculated with an agar plug near the midrib and
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Rosmarinus officinalis and Thymus vulgaris ‘Faustini’, and leaf spots of Rhododendron
‘Cunningham’s White’ were observed in three localities (private greenery, gardening center,
and ornamental nursery) in Prague, western and central Bohemia. The species Phytophthora
nicotianae Breda de Haan was consistently isolated from infected tissues on PARPNH (Jung et
al. 1996) medium. Isolates formed fluffy to slightly stellate colonies on 20% V8 agar (V8A)
plates at 25°C after 5 days, but on carrot agar plates colonies were uniform with no aerial
mycelium. The cardinal growth temperatures were: min. 8°C, optimum 27 to 32°C, and max.
37°C on V8A. Radial growth (three isolates) was 7.1 to 7.8 mm/day at 25°C. The isolates were
usually heterothallic, but the isolate from Rosmarinus sometimes in old cultures produced
smooth-walled, spherical oogonia ranging from 18.7 to 26.6 μm (n = 40) in diameter. Oospores
(14.5 to 22.8 μm in diameter, n = 40) were aplerotic with a 0.9- to 1.5-μm-thick wall.
Amphigynous antheridia averaged 11.8 × 11.7 μm (height × width, n = 40). Noncaducous
papillate (sometimes two papillae occurred) sporangia varied from ellipsoid, ovoid, pyriform, to
globose in shape and were 34.3 to 78.6 × 23.0 to 50.1 μm (n = 60). Terminal chlamydospores
(n = 60) were 13.5 to 57.5 μm with a 0.7- to 1.5-μm-thick wall. Morphological characteristics
resembled those described for P. nicotianae (Erwin and Ribeiro 1996). The isolates were
deposited in the Czech Collection of Phytopathogenic Oomycetes of RILOG Pruhonice under
nos. 1087.19, 1144.20, and 1101.19. These three isolates were sequenced for the rDNA ITS
region, partial NADH1, and partial COX1 gene. Obtained sequences were deposited in GenBank
(accession nos. MW762935 to MW762937 and MW762610 to MW762615) and compared with
sequences in the GenBank database using BLAST. The ITS sequences showed 99.7 and 100%
similarity to reference P. nicotianae sequences (KJ494902 and MH219888). The sequence of
isolate 1087.19 showed a 1-bp difference, while isolate 1144.20 contained ambiguous bases. The
COX1 and NADH1 sequences of isolates 1087.19 and 1144.20 were identical to reference P.
nicotianae sequences (accession nos. MH477752 for COX1 and AY564023 for NADH1). The
sequences of isolate 1101.19 differed in two positions in both genes and showed 99.7%
similarity to the same reference P. nicotianae sequences. The three isolates were tested for
pathogenicity on all three host species (10 plants per isolate per host species). Two-year-old R.
officinalis and T. vulgaris plants were inoculated with three 5-mm V8A plugs with mycelium
from 7-day-old colonies by inserting into the substrate near the collar. Leaves of 3-year-old
Rhododendron ‘Cunningham’s White’ were inoculated with an agar plug near the midrib and
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