© 2007 Plant Management Network.
Susceptibility of Some Lilac Cultivars and Other Members of the Oleaceae to Phytophthora ramorum
Nina Shishkoff, Research Plant Pathologist, ARS/USDA, Foreign Disease/Weed Science Research Unit, Frederick, MD 21702
Shishkoff, N. 2007. Susceptibility of some Lilac cultivars and other members of the Oleaceae to Phytophthora ramorum. Online. Plant Health Progress doi:10.1094/PHP-2007-1101-02-RS.
Lilac is a host of Phytophthora ramorum, but differences in host susceptibility of lilac cultivars and related genera have not been fully studied. This paper describes the symptoms on lilac and some other plants in the Oleaceae (Forsythia, Fraxinus, Ligustrum, and Abeliophyllum) and analyzes their relative susceptibility. Lilacs varied somewhat in susceptibility, with Syringa x josiflexa 'James MacFarlane' showing no symptoms and S. x prestoniae 'Alexander's pink' very few, but most cultivars developed large dark leaf lesions and suffered defoliation of heavily infected leaves. Fraxinus and Ligustrum were somewhat less susceptible than most lilac cultivars. The pathogen could sometimes be isolated from buds, but twig die-back was not observed. One month after the roots of Syringa, Abeliophyllum, Forsythia, and Ligustrum were inoculated, roots remained asymptomatic, but the pathogen could be recovered from washed or surface-sterilized root pieces of all genera tested except Ligustrum.
Phytophthora ramorum Werres, De Cock & Man in't Veld causes stem cankers on oaks and foliar lesions and stem dieback on a number of plants (4). It was first observed in the 1990s in European nurseries and California coastal forests (17). Although the distribution of P. ramorum in US forests is currently mainly limited to parts of California and Oregon, there is potential for spread with the movement of nursery stock. By the end of 2004, infected containerized ornamentals were inadvertently shipped from California to at least 40 states (15) leading to the instigation of an Emergency Federal Order, which placed restrictions on the movement of host plants shipped out of California, Oregon, and Washington (1). In 2003, the first infected lilacs were found in nurseries located in the United Kingdom (2). In 2004, an infected lilac was discovered in a nursery in New Jersey and in 2006 one was found in Maine (3). Since lilacs are a popular ornamental and P. ramorum sporulates abundantly on it (2), it is important to be able to recognize symptoms of the disease on a variety of cultivars. It may also be useful for plant breeders to know if there are sources of resistance. Finally, given the susceptibility of lilac, it is important to test other ornamentals or significant forest species in the Oleaceae to see if they are susceptible.
Symptoms on Above-Ground Parts
All work was conducted in containment under APHIS permit 72462 for Phytophthora ramorum. Fourteen taxa in Syringa were tested for susceptibility to Phytophthora ramorum, as well as four other genera in the family Oleaceae (Table 1). In general, the plants were rooted cuttings with 8 to 25 leaves; wild-type S. vulgaris were started from seed and inoculated as seedlings with 6 to 15 leaves. Plants were inoculated in spring, summer or early fall while leaves were still present at different stages of development. The pathogen isolate (5-C) used in these experiments was originally recovered from Camellia sasanqua 'Bonanza' in California in 2003. Sporangia in solution were prepared as described previously (14), by placing mycelial plugs in a sterile soil extract solution for 48 h. Plants were inoculated by inverting foliage and branch tips into sporangial solution (approximately 3000 sporangia/mL) and using a large paintbrush to apply the solution to any above-ground part of the plant missed by immersion. The plants were inoculated in batches of 5 to 15, with individuals of each tested species or cultivar inoculated as part of at least 2 batches (Syringa 'Miss Kim' and Abeliophyllum were inoculated in two batches; all other plants were inoculated as part of at least 4 batches). Each batch included a few rooted cuttings of Rhododendron 'Cunningham's White' used as positive controls; this cultivar develops distinct, easy-to-rate symptoms that, for the genus, are considered severe. Overall, this meant that plants were inoculated in 28 batches (approximately 10 plants per batch, 2 to 3 of which were 'Cunningham's White') over the period of 30 August 2004 to 21 August 2006 during spring, summer, and early fall.
Inoculated plants were placed in a dew chamber at 24°C for 4 days. Upon careful removal from the dew chamber (so that plants could be matched with any leaves that had fallen from them), the plants were rated for defoliation. On three occasions, drops of water were collected from the tips of 10 to 12 infected leaves of duplicate plants of S. vulgaris as they were removed from the dew chamber, and the number of sporangia in the drops counted. Buds were removed, washed, and plated on selective PARPH media (8,13). The number of diseased leaves and total leaves were counted, and each diseased leaf was rated for percent disease area. From these, one could measure disease as the number of infected plants per total inoculated, the number of infected leaves per total leaves on the plant, the proportion of infected tissue on infected leaves (roughly corresponding to "average lesion size") and the amount of infected tissue averaged over the total number of leaves on the plant. All of these measurements, taken together, give a good picture of overall susceptibility. Statistical analysis was done to compare disease severity among cultivars. Cultivars showing very little or no disease had to be excluded from data sets because the large number of zero data points made it impossible to get a normal distribution of data. Remaining data was log-transformed and analyzed by General Linear Models with Tukey's Studentized Range test using SAS (SAS Institute Inc., Cary, NC).
Infected lilacs developed dramatic symptoms: leaves had large dark water-soaked lesions with defined margins (Fig. 1). When lesions dried out, they turned brown and brittle, distorting the leaves. After 4 days lesions did not expand to encompass a whole leaf unless the leaf was young and not fully expanded. Badly diseased leaves often fell off the plant, but not to the degree seen in Camellia (10). Lilac buds were sometimes found to be infected (up to 22% of buds in the hybrid 'Royalty'), with the outer bud scales showing small dark lesions. However, while infections of leaves and branch tips in Camellia and Rhododendron often led to a twig blight as the pathogen moved down into the stem, this was not observed in lilac. Symptoms on Fraxinus (ash) (Fig. 2), Forsythia (Fig. 3), Abeliophyllum, and Ligustrum (privet) were similar.
Results of plant inoculations are summarized in Table 1. Cultivars, by definition, should have no genetic variability, and in this trial, for most of the cultivars, 100% of the plants inoculated became infected. Some of the species showed less than 100% infection, although only the S. vulgaris seedlings were known to come from seed and possibly represent a genetically diverse population. The proportion of infected leaves on a plant is an important indicator of susceptibility, although looking at this factor alone would not allow one to distinguish between a plant with many small lesions and one with many large lesions. In this test, there was not much difference in "percent infected leaves" among the cultivars and hybrids tested with the exception of the hybrids Syringa x josiflexa 'James MacFarlane' showing no foliar symptoms and S. x prestoniae 'Alexander's pink,' where few leaves became infected. Fraxinus and Ligustrum showed somewhat less susceptibility to P. ramorum than lilacs in general. The "percent lesion area/diseased leaves" showed that the proportion of diseased tissue on diseased leaves (an approximation of lesion size) did not differ very much among plants tested: ''James MacFarlane' showed no lesions and 'Alexander's pink’ had significantly smaller lesions than most of the other plants tested. Percent disease/total leaves is a good measure of the overall impact of the pathogen on the plant tested: it takes into account the total amount of disease averaged over all the leaves present. In these tests, 'James MacFarlane' showed no lesions and 'Alexander's pink,' forsythia, privet, and ash had somewhat lower disease ratings than most of the lilacs.
These results can be compared to other investigations on relative susceptibility done comparing lesion size on detached lilac leaves (5): S. vulgaris 'Tinkerbell' had the largest lesions, followed by S. patula 'Miss Kim' and S. vulgaris 'Charles Joly,' then a mass of cultivars with similar sized lesions. The smallest lesions were found on S. reticulata 'Ivory Silk,' S. vulgaris 'Mme. Lemoine,' and S. vulgaris 'Katherine Havemeyer.'
The number of sporangia collected in single drops from the tips of infected lilac leaves was 38.2 to 84.6, ranging from no sporangia seen to 640 in a single drop. If 20 drops are assumed to make up 1 ml, then there was an average sporangial concentration of 764 sporangia/mL. Granted, some of these sporangia could be left over from the original inoculation, but examination of the underside of infected leaves using a dissecting microscope showed lesions to be covered with a continuous layer of sporangia.
Infection of Roots
Seeds of S. vulgaris were grown in 3-inch pots of Turface MVP, a fired montmorillonite clay soil conditioner (Profile Products LLC, Buffalo Grove, IL), Viburnum cuttings were rooted in turface, and all other plants (S. x josiflexa 'James MacFarlane,' S. x prestoniae 'Alexander's pink,' Abeliophyllum distichum, Forsythia x intermedia, Ligustrum vulgare) were transplanted into turface when received. The two species of Viburnum were included as positive controls because Viburnum is known to have a high rate of root infection (9). The surface of the potting mix in each pot was drenched with 15 mL of a sporangial solution (approximately 3000 sporangia/mL) of P. ramorum; a few plants of each species were drenched with water only as a control. Test plants were inoculated as part of batches which always included Viburnum, and each species was inoculated as part of at least 2 batches, for a total of 9 batches. Plants were then incubated under greenhouse conditions for a month, then root samples were either washed and directly plated onto selective PARPH media, or surface-sterilized in 0.025% sodium hypochlorite for 5 to 10 min before plating.
Roots were asymptomatic in all cases, and no P. ramorum was detected on control plants. P. ramorum was recovered from S. vulgaris on both washed roots and surface-sterilized ones at 11.8 and 6.6%, respectively. That is a rate of root infection comparable to that found from inoculations of Viburnum (8.2 to 18% in washed roots, 3.5 to 3.7% in surface-sterilized roots). Because P. ramorum was isolated from surface-sterilized roots, this suggested that internal colonization of roots was occurring. A low percent root infection (0.2 to 0.3%) was seen for Syringa x josiflexa 'James MacFarlane,' which showed no foliar symptoms; this was comparable to infection of roots of plants that do not show foliar symptoms (9). Infected roots were detected at 6.0 to 13.8% for Syringa x prestoniae 'Alexander's pink,' which had low foliar disease ratings. Ligustrum had no detectable root infection, but only 4 infected plants were tested (Table 2).
Over all, there was not much difference in symptomology among lilac cultivars tested; they developed large dark leaf lesions on foliage, but did not develop stem lesions, at least in the short term. They did not differ tremendously in degree of susceptibility. The low susceptibility of Syringa x josiflexa 'James MacFarlane' and S. x prestoniae 'Alexander's pink' means that breeders have sources of resistance available through 'James Macfarlane' ('Alexander’s Pink' is a cross of 'James Macfarlane' with another lilac). However, when quarantine makes the detection of infected plants important, resistant plants with less obvious symptoms are not an advantage to growers or plant inspectors.
Detection is currently carried out by nursery inspectors looking for symptomatic plants and collecting samples. Fresh symptoms on lilac are a distinctive charcoal gray, but after leaves dry or fall off the plant there is nothing especially distinctive about them. Fallen lilac leaves can serve as a source of sporangia for a number of days, and then, as they decay, can release chlamydospores (11).
Although lilac leaves were very susceptible in artificial inoculation and lesions produce large numbers of sporangia, it has not been one of the plants commonly found infected in nursery surveys of the west coast (15). This may have to do with the interaction of plant phenology and west coast climate. Lilac is deciduous, and the buds and shoots do not appear to be very susceptible, so susceptible tissue may not be present when optimum conditions for infection occur on the west coast. The most commonly observed infected plants in nurseries on the west coast have been evergreens, Camellia and Rhododendron (15). During wet springs on the east coast, however, lilacs in nurseries might come into contact with infected stock and themselves become infected.
No other members of the Oleaceae tested were as susceptible as S. vulgaris. Fraxinus americana, a component of Eastern forests, was mildly susceptible under artificial inoculation.
Phytophthora ramorum has been found to infect roots under laboratory or greenhouse conditions (6,9,10,11,12) and such roots are often symptomless [although a close examination of Viburnum roots has shown that root tips are killed and sporulation occurs on them (unpublished data)] . Internal colonization of roots has been shown, under laboratory conditions, to lead to spread of the organism in the vascular system of the host (6). The significance of infected roots in the epidemiology of ramorum blight is unknown, but might serve as a pathway to above-ground infection, either through travel up the stem, or by inoculum splash to foliage.
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