The sugary exudate appearing on bark lesions of Persea americana Miller and Persea indica plants after infection with Phytophthora citricola contained viable oospores and hyphal fragments in the field and in the greenhouse. This sugary exudate was a source of inoculum and dispersal of the pathogen within and between avocado plants. Spraying water onto lesions moved inoculum from the sugary exudate to wounds below. Water from sprinkler irrigation washed propagules into the soil around the plants. Viable propagules of Phytophthora citricola were identified in the feces of snails (Helix aspersa) that had fed on infected bark tissues. When these snails were moved to healthy plants, they made wounds on succulent tissue, and the infectious feces induced cankers. Ants (Iridomyrmex humilis) were attracted to the sugary exudate and also transmitted infectious propagules to wounds on avocado stems and to the soil. Control strategy for the avocado stem canker disease should consider control of vectors.
At monthly intervals, plants and stem cuttings of avocado (Persea americana Miller) `Hass' grafted on `Barr Duke' rootstock and `Topa Topa' growing in a lathhouse were wounded and inoculated with the stem canker pathogen, Phytophthora citricola Sawada. The seasonal changes (measured monthly) in the extent of colonization of the avocado plants by P. citricola followed a periodic pattern, with two peaks of colonization during an annual growth cycle. Concentration of free amino acids and total soluble carbohydrates in the plant tissues followed a periodic pattern with two peaks similar to that of canker growth. Months were significantly different for canker size, free amino acids, and total soluble carbohydrates of the bark tissues. The extent of colonization was highest during May-June, after the first vegetative flush, and during November-December, after the second vegetative flush. Total free amino acids of the hark tissue was highly correlated with canker size (r = 0.89). Although the total soluble carbohydrate of the bark tissue was also elevated during the periods of canker development, it showed lower positive correlation (r = 0.45) with canker size. Plants were relatively resistant to colonization through March-April, during the first vegetative flush, and through August-September, during the second vegetative flush. Cankers formed on stem cuttings were generally larger than those of intact plants.
( Bush et al., 2003 ; Lamour et al., 2003 ; Themann et al., 2002 ). Free water significantly contributes to the dispersal of Phytophthora species and irrigation sources can serve as reservoirs for species like P. cactorum, P. cinnamomi, P. citricola
subterranean infections, whereas Phytophthoracitricola Sawada tended to invade almond scions directly, aboveground ( Browne et al., 1998 ; Browne and Viveros, 1999 ). Recently, Phytophthora niederhauserii Z.G. Abad & J.A. Abad. was described ( Abad et al
is more resistant to several species of Phytophthora ( Matheron and Mircetich, 1985b ; Mircetich and Matheron, 1983 ), but in practice this resistance has been insufficient to prevent serious losses caused by P. cinnamomi or P. citricola
Phytophthora crown and root rot (PCRR) is among the most serious diseases of Persian walnut worldwide. In California, more than 10 species of Phytophthora have been implicated in the disease, but Phytophthora cinnamomi and P. citricola are
Many nursery crops are susceptible to root and foliage diseases caused by numerous species of Phytophthora. Phytophthora ramorum causes sudden oak death of trees and ramorum leaf blight and shoot dieback on numerous nursery plants, including rhododendron (Rhododendron spp.), viburnum (Viburnum spp.), pieris (Pieris spp.), and camellia (Camellia spp.) in Europe, the United States, and British Columbia, Canada. We sought to evaluate relative susceptibility of a selection of ornamental nursery crops by inoculating detached leaves with several species of Phytophthora known to infect rhododendrons, and to compare the relative virulence on those species to isolates of P. ramorum. The results indicated that many plants were susceptible under these experimental conditions, while others were not. On a given host, symptoms caused by all species of Phytophthora were identical except for differences in pathogen virulence. Plant species were identical except for differences in pathogen virulence. Plant species within genera or cultivars within species varied in susceptibility to isolates of P. ramorum and other species of Phytophthora. Phytophthora ramorum, P. citricola, P. citrophthora, and P. nicotianae were the most virulent pathogens on most of the host plants inoculated. Some plants were susceptible to several species of Phytophthora, while others were susceptible only to P. ramorum. Inoculation of detached leaves of `Nova Zembla' rhododendron, lilac (Syringa vulgaris), or doublefile viburnum (Viburnum plicatum var. tomentosum) under controlled conditions with different species of Phytophthora or isolates of P. ramorum (both mating types) indicated significant relative differences in species or isolate virulence.
Soil solarization, alone and combined with metam sodium (MS), was evaluated as an alternative to methyl bromide and chloropicrin (MBC) fumigation, the standard soil disinfestation technique in the California strawberry (Fragaria ×ananassa Duch.) industry. Tests were conducted in two consecutive annual production cycles in Irvine, Calif., an environment representative of the coastal strawberry production area. Solarization treatments were applied from late July through September for October plantings. Treatments were equally effective in reducing baited populations of Phytophthora cactorum [(Lebert and Cohn) J. Schröt] (1989-90) and P. citricola Sawada (1990-91) when compared to pathogen survival in nontreated soil. Solarization and MBC reduced Verticillium dahliae Kleb inocnlnm in 1989-90, but MBC gave superior control in 1990-91. Solarization significantly controlled annual weeds, but was less effective than MBC. In 1989-90, solarization alone increased strawberry yield 12 % over the yield of nontreated plots; when combined with MS, yield increase was 29%, equivalent to that achieved with MBC fumigation. Treatments were equally effective in increasing yields in the 1990-91 test. Chemical names used: sodium N -methyldithiocarbamate (metam sodium), chloropicrin nitrotrichloromethane (chloropicrin).
Phytophthora ramorum, while thought to be primarily an aboveground pathogen, can be introduced into soilless potting media in the nursery industry as sporangia or chlamydospores and remain undetected while disseminated geographically. Inoculum of this pathogen, both North American (A-2 mating type) and European (A-1 mating type) isolates, was used to infest potting media components or soil, using either sporangia, chlamydospores produced in vermiculite culture, or dry infected `Nova Zembla' rhododendron (Rhododendron sp.) leaf pieces. Vermiculite chlamydospore/oospore inoculum of P. citricola, P. cactorum, and P. citrophthora were included for comparison. Survival was determined monthly by leaf disc baiting or direct plating on selective medium. Results indicated that P. ramorum survived in most media components or soil for up to 6 months when introduced as sporangia, or up to 12 months as chlamydospores. However, it was not detected at all from infected rhododendron leaf pieces by either detection method. These results show that P. ramorum can survive in potting media if introduced as sporangia or chlamydospores, and accordingly the pathogen could be disseminated geographically without being detected visually.
rot, leaf blight, dieback, and mortality. A list of plant pathogen species detected within the nursery is shown ( Table 1 ). Some root balls were infested with the Phytophthora cryptogea -complex, the Phytophthoracitricola -complex, and the