Incidence Of Lettuce Drop Research Articles

Potential for Cultural Management of Lettuce Drop (Sclerotinia sclerotiorum) in High Tunnels Through Modification of Soil Moisture, Planting Date, and Cultivar

Lettuce is a high-value crop commonly grown in high tunnels. However, lettuce drop, caused by Sclerotinia sclerotiorum, can greatly impact lettuce production and profitability. Crop loss from S. sclerotiorum can be especially severe in high tunnels because of intensive cropping and limited options for non-host rotations. A study was conducted in a Kentucky high tunnel infested with S. sclerotiorum to determine if soil moisture, planting date, and host resistance can successfully manage lettuce drop. The experiment was arranged in a split-split plot design with soil moisture (high: 60 to 100%, low: 40 to 80% plant available water), planting date (early, mid, and late spring), and lettuce cultivar (Galactic, Harmony, Pirat, and Vulcan) being the main plot, subplot, and sub-subplot factors, respectively. In year one, lettuce drop incidence was significantly lower in the low soil moisture and early planting date, and significantly higher in Pirat lettuce. Marketable lettuce weight was significantly higher in the late planting and Vulcan treatments. In year two, lettuce drop incidence was significantly lower in the late planting and Galactic compared with all other treatments. There was no significant difference in lettuce drop incidence between the high and low soil moisture in year two. Harmony and Vulcan had significantly higher marketable weight compared with all cultivars. Planting date and lettuce cultivar had the strongest effect on lettuce drop incidence. Although mid-spring is considered the ideal time to plant lettuce in Kentucky, growers managing lettuce drop should consider planting earlier or later and selecting resistant cultivars appropriate for their markets.

Effect of a High Tunnel, Organic Cropping System on Lettuce Diseases in Western Washington

Incidence of gray mold and lettuce drop, and yield of six cultivars representing market classes Boston/Crisphead, Leaf, and Romaine, were evaluated in open ended high tunnel and open field organic production systems near Mount Vernon, WA from 2010 to 2012. Each year seedlings were transplanted in April and heads harvested in June/July. In 2010, Romaine types had significantly (P < 0.0001) greater incidence of gray mold (caused by Botrytis cinerea) than other types. In 2011, incidence of gray mold was significantly (P = 0.004) greater in high tunnel than open field plots, and greatest in high tunnels when fog persisted. All cultivars were equally susceptible to lettuce drop (caused by Sclerotinia sclerotiorum), although in 2012, incidence was significantly (P < 0.0001) greater in high tunnel than open field plots. ‘Green Star’ (Leaf type) had reduced incidence of gray mold and lettuce drop in 2010 and 2011. Incidence of tipburn was significantly (P = 0.032 and P = 0.001, respectively) greater in the high tunnels in 2011 and 2012 compared to the open field. Total yield (kg) was greater in the open field in 2012, but not in 2011 and 2010. Accepted for publication 8 July 2013. Published 22 September 2013.

Impact of consumer-driven changes to crop production practices on lettuce drop caused by Sclerotinia sclerotiorum and S. minor.

Increasing demands for value-added salad products have triggered revolutionary changes in the production practices of vegetable salad crops in recent years. One of the pivotal changes is the adaptation of 2-m-wide beds for increased vegetable biomass per unit area. The move away from the traditional 1-m-wide raised beds in cool-season vegetable production and the associated irrigation practices potentially can have a major influence on diseases affecting cool-season vegetables. To assess the potential impacts of this shift on lettuce drop caused by Sclerotinia minor and S. sclerotiorum, the two bed widths and different irrigation frequencies within each were compared in two separate field experiments over four lettuce crops in 2 years. Treatments included 1- and 2-m bed widths with twice-weekly, weekly and biweekly drip irrigation serving as subplot treatments that were begun immediately following thinning. Incidence of lettuce drop was evaluated weekly thereafter until maturity. For S. sclerotiorum, 36 half-liter soil samples were also collected once each season and assayed for the number of sclerotia, and apothecia were counted weekly in a 10-m(2) area for each plot. Regardless of the species, the effects of bed width and irrigation frequency were both significant. Twice-weekly irrigation and 2-m bed width resulted in higher lettuce drop incidence than other treatments. For S. sclerotiorum, twice-weekly irrigation and 2-m bed width also significantly increased the number of apothecia per unit area and the accumulation of soilborne sclerotia over multiple cropping seasons. Results demonstrated that the 2-m bed width combined with the practiced frequency of irrigations can result in higher lettuce drop caused by S. minor and increased incidence of airborne infection by S. sclerotiorum in the Salinas Valley where, historically, it has not been a serious threat. Increased incidence of S. sclerotiorum infection in commercial lettuce fields in the Salinas Valley between 2001 and 2006 validates these experimental results. These relatively new crop production practices can alter the balance of the two Sclerotinia spp. that has long existed in California.

Interactions between Coniothyrium minitans and Sclerotinia minor affect biocontrol efficacy of C. minitans.

Coniothyrium minitans, marketed as Contans, has become a standard management tool against Sclerotinia sclerotiorum in a variety of crops, including winter lettuce. However, it has been ineffective against lettuce drop caused by S. minor. The interactions between C. minitans and S minor were investigated to determine the most susceptible stage in culture to attack by C. minitans, and to determine its consistency on S minor isolates belonging to four major mycelial compatibility groups (MCGs). Four isolates of S. minor MCG 1 and 5 each from MCGs 2 and 3 and one from MCG 4 were treated in culture at purely mycelial, a few immature sclerotial, and fully mature sclerotial phases with a conidial suspension of C. minitans. Sclerotia from all treatments were harvested after 4 weeks, air dried, weighed, and plated on potato dextrose agar for recovery of C. minitans. S. minor formed the fewest sclerotia in plates that received C. minitans at the mycelial stage; C. minitans was recovered from nearly all sclerotia from this treatment and sclerotial mortality was total. However, the response of MCGs was inconsistent and variable. Field experiments to determine the efficacy of C. minitans relative to the registered fungicide, Endura, on lettuce drop incidence and soil inoculum dynamics were conducted from 2006 to 2009. All Contans treatments had significantly lower numbers of sclerotia than Endura and unsprayed control treatments, and drop incidence was as low as in Endura-treated plots (P > 0.05). Although the lower levels of lettuce drop in Contans treatments were correlated with significantly lower levels of sclerotia, the lower levels of lettuce drop, despite the presence of higher inoculum in the Endura treatment, was attributable to the prevention of infection by S. minor. A useful approach to sustained lettuce drop management is to employ Contans to lower the number of sclerotia in soil and to apply Endura to prevent S. minor infection within a cropping season.

Effect of Sclerotium Density and Irrigation on Disease Incidence and on Efficacy of Coniothyrium minitans in Suppressing Lettuce Drop Caused by Sclerotinia sclerotiorum.

Field experiments were conducted over 2 years in Yuma, AZ, and Holtville, CA, to establish the relationship between soil sclerotium density of Sclerotinia sclerotiorum and the incidence of lettuce drop on different lettuce (Lactuca sativa) types under different irrigation systems, and to determine the efficacy of the biocontrol agent Coniothyrium minitans (Contans) against S. sclerotiorum on crisphead lettuce at varied sclerotium densities under different irrigation systems. There was no significant interaction of irrigation (overhead sprinkler versus furrow) with either sclerotium density or with biocontrol treatment. Lettuce drop incidence was lowest in romaine lettuce compared with crisphead or leaf lettuce at all soil sclerotium densities. There was a significant positive correlation between the sclerotial density and the percent disease incidence. Disease incidence in plots infested with 2 sclerotia/m2 of bed was not significantly higher than in control plots regardless of lettuce type. However, plots infested with 40 or 100 sclerotia/m2 of bed revealed a significantly higher disease incidence over the control in all lettuce types. A single application of Contans at planting significantly reduced the incidence of lettuce drop in all lettuce types even under high disease pressure. There were no significant differences between recommended (2.2 kg/ha) and high (4.4 kg/ha) application rates of Contans or between one or two applications of the product.

Efficacy of Coniothyrium minitans on lettuce drop caused by Sclerotinia minor in desert agroecosystem

Field experiments were conducted over 2 years in Yuma County, Arizona and Imperial County, California, to evaluate if increased application rates of a commercial formulation of Coniothyrium minitans (Contans) were effective against lettuce drop caused by Sclerotinia minor. The efficacy of C. minitans at varied application rates were compared to two field isolates of Paenibacillus polymyxa and the chemical fungicide Boscolid (Endura). Two applications of manufacture recommended rate of Contans (2.2 kg/ha) did not significantly reduce the incidence of lettuce drop caused by S. minor, even though applications at this rate have been shown to be very effective in controlling lettuce drop caused by Sclerotinia sclerotiorum. However, two applications of high rates of Contans (6.6, 8.8, or 11 kg/ha), one at planting and one at post-thinning, significantly reduced the incidence of lettuce drop in most trials. Two isolates of P. polymyxa each applied at a rate of 9.4 L/ha (10 9 cfu/ml) were not effective in reducing the incidence of lettuce drop. Two applications of Endura, one at thinning and one at 4 weeks post-thinning, significantly reduced the incidence of lettuce drop in Yuma County, AZ, but not in Imperial County, CA. In summary, successful management of lettuce drop caused by S. minor in desert ecosystem could best be achieved with high application rates of C. minitans.

Assessment of Resistance in Lettuce (Lactuca sativa L.) to Mycelial and Ascospore Infection by Sclerotinia minor Jagger and S. sclerotiorum (Lib.) de Bary

Lettuce drop caused by Sclerotinia spp. is an economically important disease of lettuce (Lactuca sativa L.), and cultivars with resistance to mycelial infection by Sclerotinia sclerotiorum (Lib.) de Bary and S. minor Jagger as well as to S. sclerotiorum ascospores are needed. Assessing resistance in field experiments can be complicated by fast bolting or small stature lettuce lines that may escape rather than resist the pathogens. Therefore, methods to select resistant lines from morphologically variable populations are needed. We used S. sclerotiorum and S. minor-infested field experiments, regression analysis, field experiments with artificially high plant densities, and S. sclerotiorum ascospore inoculations to identify lettuce lines with resistance to both pathogens. Three replicated experiments in S. sclerotiorum-infested fields were conducted in Yuma, AZ, and three replicated experiments in a S. minor-infested field were conducted in Salinas, CA, using diverse populations of iceberg, romaine, leaf, butterhead, Latin, oilseed lettuce, and wild relatives of lettuces; and genetic variation for the incidence of lettuce drop from mycelial infections was identified. In two S. minor field experiments, a quadratic regression model was developed that related rapid bolting with reduced lettuce drop. Regression residuals were calculated, and eight cultivars or PIs had negative residuals in two independent field experiments, indicating higher resistance than predicted by their rate of bolting. Eruption, a small-statured Latin cultivar, had significantly lower disease levels than susceptible cultivars in experiments with high plant densities, indicating that its small size did not facilitate disease escape. Ascospore inoculations confirmed resistance in ‘Eruption’ and L. virosa SAL012, whereas the oilseed lettuce PI 251246 may have partial resistance to infection. These lines will likely be useful for development of Sclerotinia spp.-resistant lettuce cultivars.

Mustard and Other Cover Crop Effects Vary on Lettuce Drop Caused bySclerotinia minorand on Weeds

Mustard cover crops have been suggested as a potential biofumigant for managing soilborne agricultural pests and weeds. We conducted several experiments in commercial lettuce fields in the Salinas Valley, CA, to evaluate the effects of mustard cover crops on lettuce drop caused by Sclerotinia minor and on weed density and seed viability. In a long-term study, we measured the effects of white and Indian mustard cover crops on the density of S. minor sclerotia in soil, lettuce drop incidence, weed densities, weed seed viability, and crop yield in head lettuce. We also tested broccoli and rye cover crop treatments and a fallow control. Across several short-term studies, we evaluated the density of S. minor sclerotia in soil, lettuce drop incidence, weed densities, and weed seed viability following cover cropping with a mustard species blend. Numbers of sclerotia in soil were low in most experimental locations and were not affected by cover cropping. Mustard cover crops did not reduce disease incidence in the long-term experiment but the incidence of lettuce drop was lower in mustard-cover-cropped plots across the short-term experiments. With the exception of common purslane and hairy nightshade, weed densities and weed seed viability were not significantly reduced by cover cropping with mustard. Head lettuce yield was significantly higher in mustard-cover-cropped plots compared with a fallow control. Glucosinolate content in the two mustard species was similar to those measured in other studies but, when converted to an equivalent of a commercial fumigant, the concentrations were much lower than the labeled rate for lettuce production. Although mustard cover cropping resulted in yield benefits in this study, there was little to no disease or weed suppression.

Biocontrol of Lettuce Drop Caused by Sclerotinia sclerotiorum and S. minor in Desert Agroecosystems.

Field experiments were conducted over 2 years in Yuma County, AZ, and Imperial County, CA, to determine the efficacy of several biocontrol agents for the management of lettuce drop caused by Sclerotinia spp. Commercial formulations of Trichoderma harzianum (Plantshield, Supersivit), Gliocladium virens (Soilgard), Coniothyrium minitans (Contans), and Bacillus subtilis (Companion) were evaluated and compared with the chemical fungicide iprodione (Rovral) against Sclerotinia sclerotiorum and S. minor. A single application of biocontrol products or of Rovral did not reduce lettuce drop caused by either Sclerotinia species. However, two applications of Contans, one at planting and one at post-thinning, significantly reduced the incidence of lettuce drop caused by S. sclerotiorum and increased yield but had no effect on S. minor at both locations in both years. Two applications of other biocontrol products did not significantly reduce disease incidence despite medium to high recovery following application. In contrast, Contans was only sporadically recovered following application. In vitro fungicide sensitivity evaluation revealed that both Trichoderma and Gliocladium species were tolerant to iprodione, dicloran (Botran), and vinclozolin (Ronilan) up to 1,000 ppm a.i., whereas both Sclerotinia spp. and C. minitans were sensitive to all three fungicides above 1 ppm. In summary, Contans was the most effective treatment for the control of lettuce drop caused by S. sclerotiorum, but no treatment was effective against S. minor in the desert lettuce production systems.

Analyses of Lettuce Drop Incidence and Population Structure ofSclerotinia sclerotiorumandS. minor

ABSTRACT 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 field-to-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, approximately 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.

Dynamics of Lettuce Drop Incidence and Sclerotinia minor Inoculum Under Varied Crop Rotations.

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 cm3 of soil) that was distributed randomly, included: continuous lettuce (LLL), lettuce rotated with broccoli (LBL), and lettuce followed by a fallow period (LFL). Treatments at the Hartnell site, which had a high density of inoculum (>7 sclerotia per 100 cm3 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 3 for 100% infection and 200 cm3 for 50% infection. Thus, in 100 cm3 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.

Comparative Analyses of Lettuce Drop Epidemics Caused by Sclerotinia minor and S. sclerotiorum.

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 I) and those caused by the airborne ascospores (type II). 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.

Solarization and fungicides for the control of drop, bottom rot and weeds in lettuce

Soil solarization was applied for 60 days during the summer in lettuce fields in Mogi das Cruzes (latitude 22°31′S, longitude 48°11′W), southeastern Brazil, from December to January, 1998/99 and 1999/2000, after which two consecutive lettuce crops were grown in each cropping season. Soil solarization was tested in association with the fungicides pencycuron and procymidone, with the purpose of controlling bottom rot and lettuce drop, caused by Rhizoctonia solani AG1-I and Sclerotinia minor, respectively. In the first crop of both years the solarized treatments presented very low incidences of the diseases and shortened the crop cycle by 10 days as compared to the non-solarized plots. In both years the incidence of lettuce drop in the second (winter) crops was very low in the solarized plots, but a 25–40% loss of lettuce heads to the disease was observed in the control plots (no solarization and no fungicides). Although the incidence of diseases in the non-solarized plots was reduced by the fungicides, they did not offer additional benefits in the solarized areas in both crops. Reductions in biomass C and in the number of fungi and bacteria were observed in the solarized treatments, but the amount of fluorescent pseudomonas was not affected by the soil treatments. The concentrations of soil nutrients NH 4–N and DTPA-extractable-Mn increased sharply due to solarization. Changes in soil nutrient availability and probably in the microbial community, among other non-detected benefits, may be related to the reduction of the crop cycle in the present work.

Relationship Between Plant Morphological Traits and Resistance to Sclerotinia minor in Lettuce

A low to moderate incidence of lettuce drop caused by Sclerotinia minor is commonly observed in commercial lettuce fields of all types of lettuce (e.g. crisphead, romaine, leaf, butter) and although partial resistance has been reported, no sources of immunity have been described. We sought to determine whether there was variability between different types of lettuce and among cultivars within types. Replicated experiments were conducted in an infested field using established inoculation procedures. Significant variation in susceptibility to S. minor was detected among cultivars within as well as between major lettuce types. Correlations between lettuce drop susceptibility and plant canopy size, seedling vigor, and additional morphological traits were determined. Variability between different field experiments was also evaluated for several traits and a subset of cultivars. Our results suggest that cultivated germplasm may provide genes that are as useful or more useful than those found in genotypes with more primitive growth habits in developing cultivars with tolerance to lettuce drop.

Effects of Irrigation and Tillage on Temporal and Spatial Dynamics of Sclerotinia minor Sclerotia and Lettuce Drop Incidence

ABSTRACT 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.

Sclerotinia Stem and Crown Rot of Corn-Salad Caused by Sclerotinia minor in California.

Corn-salad or lamb's lettuce (Valerianella locusta) is a specialty leafy green vegetable that is grown commercially in California and is harvested fresh for use in salads. In 2001, field plantings of corn-salad in coastal California showed symptoms and signs of a previously undescribed disease. Initial symptoms consisted of a light tan discoloration at the crown and lower leaf attachment areas. Once this discoloration was observed, the crown rapidly developed a soft rot, attached leaves wilted, and the entire plant collapsed. White mycelium and small (0.5 to 3.0 mm in diameter), irregularly shaped, black sclerotia formed on the crowns and lower leaves. Isolations from symptomatic crowns, mycelium, and sclerotia produced colonies of Sclerotinia minor (1). Seven-week-old corn-salad plants grown in a peat moss-based rooting medium in pots were used to test pathogenicity. Sclerotia from six corn-salad isolates from the Salinas Valley were inserted into slots made in the potting mix adjacent to the crowns of plants. Sclerotia were not placed in slots for control corn-salad. All test plants were incubated in a greenhouse at 21 to 23°C. After 4 weeks, inoculated corn-salad plants wilted and collapsed, and S. minor was reisolated from necrotic crown and stem tissues. Uninoculated plants were asymptomatic. Using the same method, sclerotia from one lettuce (Lactuca sativa) isolate were used to inoculate corn-salad plants that produced similar symptoms. All experiments were repeated and results were similar. To our knowledge, this is the first report of corn-salad as a host of S. minor in California and the United States. The susceptibility of corn-salad to S. minor from lettuce indicates that this crop might contribute to inoculum levels and lettuce drop incidence for the extensive lettuce plantings in the Salinas Valley. Reference: (1) C. L. Patterson and R. G. Grogan. Plant Dis. 72:1046, 1988.

Effects of Broccoli Rotation on Lettuce Drop Caused by Sclerotinia minor and on the Population Density of Sclerotia in Soil.

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 (<7 sclerotia per 100 cm3 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 100 cm3 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.

Organic Amendment Effects on Survival and Incidence of Lettuce Drop Caused by Sclerotinia minor

ABSTRACT To assess the effect of three organic amendments on incidence of lettuce drop caused by Sclerotinia minor Jagger on Lactuca sativa L., a disease incidence test, two sclerotia survival tests and associated mycoflora study were conducted. Meat powder, chicken manure and alfalfa hay were compared at two concentrations each: 2.5 and 7.5% w/w. Incidence of wilted plants and standardized area under disease progress curve were recorded in the incidence test, while recovery and proportion of destroyed, non-viable and parasitized sclerotia were assessed in sclerotia survival tests. In the mycoflora study, quantitative and qualitative aspects of the fungal population of the most effective treatments were recorded in order to assess their role on suppressiveness. The reduction of disease parameters by means of organic amendment verified in this work is shown to be dose-dependent. The final incidence was significantly lower than that recorded in the inoculated controls only when alfalfa hay and chicken manure were applied at the greatest dose (7.5% w/w). The results obtained in the present work do not establish a firm association between biological control and the suppression observed when alfalfa hay and chicken manure are used as amendments.

Mechanisms of Subsurface Drip Irrigation-Mediated Suppression of Lettuce Drop Caused by Sclerotinia minor

ABSTRACT Subsurface drip irrigation and associated mandatory minimum tillage practices significantly reduced the incidence of lettuce drop (Sclerotinia minor) and the severity of corky root on lettuce compared with furrow irrigation and conventional tillage. Three possible mechanisms for the drip irrigation-mediated disease suppression were examined in this study: qualitative and quantitative differences in the soil microflora under furrow and subsurface drip irrigation; their antagonism and potential bio-control effects on S. minor; and the physical distribution of soil moisture and temperature relative to the two irrigation methods. To determine if the suppressive effects under subsurface drip irrigation were related to changes in soil microflora, soils were assayed for actinomycetes, bacteria, and fungi during the spring and fall seasons. The effects of the irrigation methods on microbial populations were nearly identical during both seasons. In the spring season, the total number of fungal colonies recovered on potato dextrose agar amended with rose Bengal generally was greater in soils under drip irrigation than under furrow irrigation, but no such differences were observed during the fall. Numbers of actinomycetes and bacteria were not significantly different between irrigation methods during either season. No interaction between sampling time and irrigation methods was observed for any of the microbial populations during both seasons. Thus, the significant effect of sampling time observed for actinomycete and bacterial populations during the spring was most likely not caused by the irrigation treatments. There were also no qualitative differences in the three groups of soil microflora between the irrigation treatments. Even though some fungal, actinomycete, and bacterial isolates suppressed mycelial growth of S. minor in in vitro assays, the isolates came from both subsurface drip- and furrow-irrigated soils. In in planta assays, selected isolates failed to reduce the incidence of drop in lettuce plants. The soil moisture under subsurface drip irrigation was significantly lower at all depths and distances from the bed center after an irrigation event than under furrow irrigation. The soil temperature, in contrast, was significantly higher at both 5 and 15 cm depths under drip irrigation than under furrow irrigation. The suppression of lettuce drop under subsurface drip irrigation compared with furrow irrigation is attributed to differential moisture and temperature effects rather than to changes in the soil microflora or their inhibitory effects on S. minor.

Comparison of Lettuce Diseases and Yield Under Subsurface Drip and Furrow Irrigation

ABSTRACT Subsurface drip and furrow irrigation were compared on lettuce (Lactuca sativa) cvs. Salinas and Misty Day for yield and incidence and severity of three important diseases of lettuce in the Salinas Valley, CA. Experiments were conducted between 1993 and 1995 during the spring and fall seasons. The diseases examined included lettuce drop (Sclerotinia minor), downy mildew (Bremia lactucae), and corky root (Rhizomonas suberifaciens). Replicated plots of subsurface drip and furrow irrigation were arranged in a randomized complete-block design. All plants were inoculated with S. minor at the initiation of the experiment during the 1993 spring season. Plots were not inoculated for downy mildew and corky root during any season nor were the plots reinoculated with S. minor. During each season, all plots were sprinkler irrigated until thinning, and subsequently, the irrigation treatments were begun. The furrow plots were irrigated once per week, and the drip plots received water twice per week. The distribution of soil moisture at two soil depths (0 to 5 and 6 to 15 cm) at 5, 10, and 15 cm distance on either side of the bed center in two diagonal directions was significantly lower in drip-irrigated compared with furrow-irrigated plots. Plots were evaluated for lettuce drop incidence and downy mildew incidence and severity at weekly intervals until harvest. Corky root severity and yield components were determined at maturity. Lettuce drop incidence and corky root severity were significantly lower and yields were higher in plots under subsurface drip irrigation compared with furrow irrigation, regardless of the cultivar, except during the 1994 fall season. Incidence and severity of downy mildew were not significantly different between the two irrigation methods throughout the study. The differential microclimates created by the two irrigation treatments did not affect downy mildew infection, presumably because the mesoclimate is usually favorable in the Salinas Valley. Subsurface drip irrigation is a viable, long-term strategy for soilborne disease management in lettuce in the Salinas Valley.