To understand the geographical distribution of lettuce drop incidence and the structure of Sclerotinia minor and S. sclerotiorum populations, commercial lettuce fields were surveyed in the Salinas, San Joaquin, and Santa Maria Valleys in California. Lettuce drop incidence, pathogen species, and mycelial compatibility groups (MCGs) were determined and analyzed using geostatistic and geographical information system tools. Lettuce drop incidence was lowest in the San Joaquin Valley, and not significantly different between the other two valleys. Semivariogram analysis revealed that lettuce drop incidence was not spatially correlated between different fields in the Salinas Valley, suggesting negligible fieldto-field spread or influence of inoculum in one field on other fields. Lettuce drop incidence was significantly lower in fields with a surface drip system than in fields with furrow or sprinkler irrigation systems, suggesting that the surface drip system can be a potential management measure for reducing lettuce drop. In the San Joaquin Valley, S. sclerotiorum was the prevalent species, causing drop in 63.5% of the fields, whereas S. minor also was identified in 25.4% of the fields. In contrast, in the Salinas Valley, S. minor was the dominant species (76.1%) whereas S sclerotiorum only observed in only 13.6% fields, in which only a few plants were infected by S. sclerotiorum. In the Santa Maria Valley, both species frequently were identified, with S. minor being slightly more common. Although many MCGs were identified in S. minor, most of them consisted of only one or two isolates. In all, ≈91.4% of the isolates belonged to four MCGs. Among them, MCG-1 was the most prevalent group in all three valleys, accounting for 49.8% of total isolates. It was distributed all over the surveyed areas, whereas other MCGs were distributed more or less locally. Populations of S. sclerotiorum exhibited greater diversity, with 89 isolates collected from the Salinas and San Joaquin Valleys belonging to 37 different MCGs. Among them, the most recurrent MCG-A contained 16 isolates, and 30 MCGs contained only 1 isolate each. Many MCGs occurred within only one or a part of the two valleys. Potential reasons for this abundant diversity are discussed.

The temporal and spatial dynamics of Sclerotinia minor sclerotia and the resulting incidence of lettuce drop were studied under furrow irrigation with conventional tillage and subsurface-drip irrigation with minimum tillage during 1993-95. Lettuce crops were grown each year during the spring and fall seasons. All plants were inoculated immediately after thinning in the spring of 1993. Grids of 24 contiguous quadrats (1 by 1 m(2)) were demarcated in the centers of each 150-m(2) plot. Lettuce drop incidence in each quadrat was evaluated each season prior to harvest. One soil sample (100 cm(3)) was collected from each quadrat at harvest and after tillage prior to planting of the next crop for both spring and fall crops and assayed for S. minor sclerotia using wet sieving. Lloyd's index of patchiness, the beta-binomial distribution, and variance of moving window averages were used to evaluate the spatial patterns of sclerotia and lettuce drop incidence under the two irrigation systems and associated tillage treatments. Disease incidence remained significantly higher under furrow irrigation than under subsurface-drip irrigation throughout the study period, and was significantly higher on fall crops than on spring crops. Under furrow irrigation, the number of sclerotia at the end of a crop season increased significantly over that at the beginning of the season, but no significant changes were detected over years. In contrast, the number of sclerotia within a single season did not increase significantly under subsurface drip irrigation, nor was year-to-year accumulation of sclerotia statistically significant. The degree of aggregation of sclerotia increased significantly during a cropping season under furrow irrigation, but not under subsurface drip irrigation. The conventional tillage after harvest under furrow irrigation decreased the degree of aggregation of sclerotia after each season, but the distribution pattern of sclerotia under subsurface-drip irrigation changed little by the associated minimum tillage. Spatial pattern analyses suggested that the aggregation of S. minor sclerotia occurred at a scale of no more than 1 m, and distribution of diseased lettuce plants was random at a scale larger than 1 m. The combination of fewer sclerotia produced by each crop and its unaltered distribution under subsurface drip irrigation and associated minimum tillage makes it a valuable cultural practice for lettuce drop management.

Field experiments were conducted to determine the population dynamics of Sclerotinia minor and incidence of lettuce drop at two sites during 1995 to 1998. Rotation treatments at the Spence site, which had a low density of inoculum ( 7 sclerotia per 100 cm(3) of soil) that was distributed uniformly, included: continuous lettuce (LLLL), alternate crops of broccoli and lettuce (BLBL), continuous broccoli or lettuce (BBLL), and fallow-lettuce-fallow-lettuce (FLFL). Under continuous lettuce cropping (LLLL) at the Hartnell site, a progressively aggregated spatial pattern of inoculum distribution developed, despite the initial uniform distribution of high inoculum density. In the fallow treatment (FLFL), the spatial pattern tended to be aggregated following a lettuce crop and less aggregated or random when left fallow. In contrast to these two treatments, treatments involving rotations with broccoli (BLBL and BBLL) exhibited consistently random spatial patterns of inoculum regardless of the crop in the field. The marginal increases in the number of sclerotia contributed by the few diseased lettuce plants were offset by the significant reductions in the number of sclerotia by the broccoli residue. Spatial patterns of disease incidence reflected the pattern of inoculum distribution in the soil at the Hartnell site. Higher inoculum density coupled with an aggregated distribution was associated with an aggregation in disease incidence. At Spence, this correlation was poor in most seasons because of progressive decline in the lettuce drop incidence and lack of treatment differences. In greenhouse experiments, the competence volume for S, minor sclerotia was quantified, which was calculated to be 25 cm(3) for 100% infection and 200 cm(3) for 50% infection. Thus, in 100 cm(3) of soil, a minimum of four to five sclerotia are needed for 100% of infection, explaining the high correlation between inoculum density and disease incidence.

Temporal and spatial patterns of lettuce (Lactuca sativa) drop caused by Sclerotinia minor and S. sclerotiorum were determined in lettuce fields in the Salinas, Santa Maria, and San Joaquin Valleys in California during 1995 to 1998. Of the 25 commercial fields assessed, 14 had predominantly S. minor. 9 had predominantly S. sclerotiorum, and 2 had varying levels of both species. Sclerotinia infections were classified based on symptoms: those caused directly by eruptive germination of sclerotia (type 1) and those caused by the airborne ascospores (type 11). The precise location of diseased and healthy plants was mapped and lettuce drop progress was determined at different crop growth stages. Spatial patterns of disease incidence were analyzed using 1-by-1-, 2-by-2-, 3-by-3-. and 4-by-4-m quadrat sizes. Regardless of the analytical method employed, disease incidence with type I infection showed an aggregated pattern in a majority of the fields evaluated and random patterns in fields where incidence was low. In all fields with type I infection, disease progress followed the monomolecular model, typical of soilborne diseases. For fields with aggregated distribution, spatial dependence was observed up to 10 m and was either isotropic or random in direction, suggesting the potential influence of tillage operations on inoculum distribution and disease incidence. Lettuce drop incidence in fields with type II infection was erratic in time and peaked within a very short time. However, disease incidence showed an aggregated pattern in all fields evaluated. Spatial dependence of quadrats generally was detected in two adjacent directions, suggesting a directional gradient perhaps caused by wind direction during ascospore dissemination. Increasing quadrat sizes usually increased the degree of aggregation of lettuce drop, but not the distribution pattern itself. These results demonstrate that the source of inoculum and the type of infections they cause are most likely to determine spatial patterns of lettuce drop in the field.

Polygalacturonic acid (PGA) is an important constituent of Sorensen's NP-10 medium (NP-10) for estimating the population density of Verticillium dahliae in soil. Different types of PGA are available, but not all of them favor the growth of V dahliae. Unavailability of PGA sodium salt from orange (P-1879) has created an unprecedented problem for the quantification of microsclerotia (MS) of V dahliae in soil. The PGA from orange (P-3889) that is now available does not support the growth of V dahliae. Therefore, experiments were conducted to optimize the use of NP-10 prepared with P-3889 and various concentrations of NaOH. NP-10 with P-3889 amended with eight concentrations of NaOH were compared with NP-10 prepared from PGA sodium salt from orange (P-1879, now discontinued) and citrus (P-3850) along with cellophane and Na-pectate media for recovery of MS from soil and growth of V dahliae on the media. Seven soils were assayed for MS, and eight isolates of V dahliae were evaluated for growth and production of MS. Concentrations of NaOH >0.035N and <0.02N in NP-10 with P-3889 reduced mycelial growth, microsclerotial production, and recovery of MS from soils. Similarly, NP-10 with P-3850 alone, cellophane, and Na-pectate media had significantly reduced growth on media and recovery of V dahliae from soils. The NP-10 with P-3889 and 0.025N NaOH consistently yielded numbers of V dahliae MS from soil samples and supported the growth and production of MS similar to the NP-10 with P-1879. The medium developed in this study can serve as a direct replacement for the original NP-10 that was developed nearly three decades ago, an important component of which is no longer available.

Sclerotial germination of three isolates each of Sclerotinia minor and S. sclerotiorum was compared under various Soil moisture and temperature combinations in soils from Huron and Salinas, CA. Sclerotia from each isolate in soil disks equilibrated at 0, -0.03, -0.07, -0.1, -0.15, and -0.3 MPa were transferred into petri plates and incubated at 5, 10, 15, 20 25, and 30degreesC. Types and levels of germination in the two species were recorded. Petri plates in which apothecia were observed were transferred into a growth chamber at 15degreesC with a 12-h light-dark regime. All retrievable sclerotia were recovered 3 months later and tested for viability. Soil type did not affect either the type or level of germination of sclerotia. Mycelial germination was the predominant mode in sclerotia of S. minor, and it occurred between -0.03 and -0.3 MPa and 5 and 25degreesC, with an optimum at -0.1 MPa and 15degreesC. No germination occurred at 30degreesC or 0 MPa. Soil temperature. moisture, or soil type did not affect the viability of sclerotia of either species. Carpogenic germination of S. sclerotiorum sclerotia, measured as the number of sclerotia producing stipes and apothecia, was the predominant mode that was affected significantly by soil moisture and temperature. Myceliogenic germination in this species under the experimental conditions was infrequent. The optimum conditions for carpogenic germination were 15degreesC and -0.03 or -0.07 MPa. To study the effect of sclerotial size on carpogenic germination in both S. minor and S. sclerotiorum, sclerotia of three distinct size classes for each species were placed in soil disks equilibrated at -0.03 MPa and incubated at 15degreesC. After 6 weeks, number of stipes and apothecia produced by sclerotia were counted. Solitary S. minor sclerotia did not form apothecia, but aggregates Of attached sclerotia readily formed apothecia. The number of stipes produced by both S. minor and S. sclerotiorum was highly correlated with sclerotial size. These results suggest there is a threshold of sclerotial size below which apothecia are not produced, and explains, in part, why production of apothecia in S. minor seldom occurs in nature.

Field experiments were conducted at Spence Road site and at the Hartnell College East Campus site in Salinas, CA, to determine the effects of crop rotation with broccoli or a fallow period on lettuce drop caused by Sclerotinia minor and the density of pathogen sclerotia in the soil. Treatments at the Spence Road site with low inoculum density (cm(3) Of Soil) distributed randomly included: successive crops of lettuce (LLL), lettuce rotated with broccoli (LBL), and lettuce followed by a fallow period (LFL). Treatments at the Hartnell site with high inoculum density (>7 sclerotia per too cm(3) Of Soil) distributed uniformly included: continuous lettuce (LLLL), broccoli-lettuce-broccoli-lettuce (BLBL), broccoli-broccoli-lettuce-lettuce (BBLL), and fallow-lettuce-fallow-lettuce (FLFL). At the Spence Road site, continuous lettuce did not increase lettuce drop incidence for at least 2 years, although an increase in soilborne sclerotia was observed annually but was below the threshold at which a correlation between inoculum density and disease incidence is observed. Rotation with broccoli resulted in small reductions in disease incidence only in the first year. The density of sclerotia was lowest in the LFL treatment, and the highest in the LLL. At the Hartnell site, rotation with broccoli significantly reduced both sclerotia and lettuce drop incidence. The number of broccoli crops rather than the sequence of lettuce rotations with broccoli was critical for reducing the numbers of S. minor sclerotia in soil. Fallowing after a lettuce crop resulted in marginal reductions in sclerotia and lettuce drop incidence. Viability of recovered sclerotia was not significantly different between treatments, although differences between seasons were detected. Results suggest that rotations with broccoli can be a practical lettuce drop management strategy.

The effects of temperature (5 to 25degreesC), relative humidity (81 to 100%), wind speed (0 to 1.0 in s(-1)), and their interactions on sporulation of Bremia lactucae on lettuce cotyledons were investigated in controlled conditions. Sporulation was affected significantly (P 0.5 in s(-1), regardless of RH. In still air, the number of sporangiophores produced per cotyledon increased linearly with RH from 81 to 100% (P = 0.0001, r = 0.98). Histological observations indicated that sporulation may be affected by stomatal aperture in response to RH, as more closed stomata and correspondingly fewer sporangiophores were present at lower RE. These results are important for understanding the mechanism of RE effects on sporulation and for predicting conditions conducive to downy mildew development.

Beginning in 2000 and continuing into 2004, a previously undescribed disease caused significant damage to romaine lettuce (Lactuca sativa) in the coastal Salinas Valley of California. Symptoms were brownish black, sunken cavities on the crown and upper taproot. Cavities were firm, lacked signs of fungal growth, and resulted in cracking and weakening of the crown. Affected plants were stunted, uneven in growth, and unmarketable. Crisphead lettuce also developed these symptoms, although disease incidence was always lower than that for romaine. A fungus was consistently isolated from cavities. Using morphological features and molecular methods, the fungus was identified as Phoma exigua. Pathogenicity of isolates from romaine and crisphead lettuce was demonstrated on both of these lettuce types. Replicated field studies showed that azoxystrobin, boscalid, and cyprodinil + fludioxonil fungicides effectively controlled the disease. This is the first report of a crown disease caused by P. exigua in the United States. A similar disease, Phoma basal rot on greenhouse grown lettuce in the United Kingdom, had been previously observed, but no report has been published.

A new method for analysing spatial patterns was designed based on the variance of moving window averages (VMWA), which can be directly calculated in geographical information systems or a spreadsheet program (e.g. MS Excel). Different types of artificial data were generated to test the method. Regardless of data types, the VMWA method correctly determined the mean cluster sizes. This method was also employed to assess spatial patterns in historical plant disease survey data encompassing both airborne and soilborne diseases. The results obtained using the VMWA method were generally different from those obtained with Lloyd's index of patchiness and beta-binomial distribution methods, were in partial agreement with the results from spatial analysis by distance indices, and were highly consistent with the results from semivariogram and spatial autocorrelation analysis methods. Results demonstrated that the VMWA method can be applied to many types of data, including binomial diseased or healthy plant counts, incidence, severity, and number of diseased plants or pathogen propagules although directional and edge effects may limit its application.