Epidemiological Characterization of Lettuce Drop and Biophysical Features of the Host Identify Soft Stem as a Susceptibility Factor to Sclerotinia minor

The soilborne fungus Sclerotinia minor was not known to produce sclerotia in the stems of infected and uncollapsed Lactuca standing intact until our observation in a greenhouse in 2017. We investigated lettuce–environment–S. minor interactions in two tolerant and four susceptible Lactuca genotypes to determine putative risk factors and targets for disease control. Symptomatological, pathophysiological, developmental, basal stem biophysical, and microclimate responses (27 variables) of the genotypes were determined under field and/or greenhouse conditions. Distinct patterns of infection responses were observed between modern cultivars and their primitive/wild relatives. The modern cultivars were susceptible to rapid basal stem and root degradations by S. minor. The oil-seed lettuce PI 251246 and the wild L. serriola 11-G99 were resilient to degradations and significantly deterred mycelium emergence and symptom development, but sclerotia formed to a significantly higher height in their stems. Photosynthetic efficiency declined rapidly within 1-day postinoculation (dpi) in susceptible plants but remained intact ~5–6 dpi in the tolerant 11-G99. Stomatal conductance spiked rapidly in 11-G99 plants within 1–3 dpi, coinciding with the emergence of fungal mycelia at the crown. A strong negative correlation detected between basal stem degradation severity/collapse and stem mechanical strength indicated that stem strength-mediated genetic factors determine the outcome of Sclerotinia infections of the host. Soft stem is a prominent lettuce drop susceptibility factor that could be targeted in resistance breeding. It also provides the prelude for the analysis of the biological basis of plant architecture-mediated resistance to Sclerotinia spp. in lettuce and other hosts.

Optimizing Fungicide Inputs for Management of Lettuce Drop Caused by Sclerotinia minor and S. sclerotiorum

Lettuce drop, caused by the soilborne fungi Sclerotinia minor and S. sclerotiorum, continues to be an important disease on this crop in Arizona. Trials were conducted over a 5-year period to compare different fungicides as well as the number, timing, and method of application. Compared with nontreated plots, disease reduction ranging from 49.6 to 61.0% was achieved on lettuce beds containing S. minor and treated with fluopyram + trifloxystrobin, fluazinam, fluxapyroxad + pyraclostrobin, and boscalid. Treatment of beds containing S. sclerotiorum with Coniothyrium minitans, iprodione, and boscalid reduced lettuce drop from 50.6 to 71.5%. No difference in disease control was noted between one and two applications of boscalid in plots containing either pathogen. In the presence of S. minor, beginning the first of two applications of boscalid after seeding did not differ from starting after thinning; however, in plots containing S. sclerotiorum, starting application after seeding was superior to beginning after thinning. Physical incorporation of soil treated with boscalid to a depth of 5.0 cm did not differ from soil treatment without incorporation in plots containing S. minor or S. sclerotiorum. On the other hand, in beds containing S. sclerotiorum, incorporation of soil treated with iprodione improved disease control compared with no incorporation.

Compatibility of Trichoderma isolates with pesticides used in lettuce crop

ABSTRACT Lettuce drop, caused by Sclerotinia minor and S. sclerotiorum, is one of the most important diseases that affect lettuce crop in Brazil. In previous studies, isolates of Trichoderma asperellum (IBLF 897, IBLF 904 and IBLF 914) and T. asperelloides (IBLF 908) were selected for the biocontrol of this disease. In this subsequent study, the compatibility of these isolates with pesticides used in lettuce crop in Brazil was evaluated. Initially, the mycelial growth of isolates was evaluated in culture medium plus pesticides. Then, the effect of pesticides on the parasitism of T. asperelloides isolate IBLF 914 in baits and sclerotia of S. minor and S. sclerotiorum, as well as on the survival of lettuce seedlings, was evaluated in gerboxes after application on baits and sclerotia of the antagonist with pesticides at their respective commercial doses. The fungicides pencycuron and mandipropamid and the insecticide imidacloprid did not affect the mycelial growth of Trichoderma isolates. The fungicide iprodione did not affect the mycelial growth of T. asperellum isolates. but the isolate of T. asperelloides was sensitive from the concentration of 10 µg.L-1 fungicide. Procymidone reduced the mycelial growth of Trichoderma isolates from the concentration of 10 µg.L-1 fungicide, and azoxystrobin reduced the conidial germination of the isolates of the antagonist, showing LD50 between 0.36 and 0.42 µg.L-1 fungicide. On the other hand, in the experiment carried out in “gerboxes”, none of the pesticides reduced the parasitism of baits and sclerotia or reduced the control of S. minor and S. sclerotiorum in lettuce seedlings. Results indicate that the biological control of lettuce drop with T. asperelum isolate IBLF 914 can be compatible with the remaining phytosanitary treatments used in lettuce crop.

Effect of temperature on mycelial growth of Trichoderma, Sclerotinia minor and S. sclerotiorum, as well as on mycoparasitism

ABSTRACT Environmental conditions are very important for the biological control of plant diseases. In a previous study, isolates of Trichoderma asperellum (IBLF 897, IBLF 904 and IBLF 914) and T. asperelloides (IBLF 908) were selected as antagonists of S. minor and S. sclerotiorum, causal agents of lettuce drop, one of the most relevant diseases affecting the lettuce crop. In this subsequent study, the mycelial growth of these isolates and pathogens, as well as the mycoparasitism of isolate IBLF 914, was evaluated at different temperatures. The mycelial growth of the isolates of T. asperellum and T. asperelloides, as well as of S. minor and S. sclerotiorum, was evaluated at temperatures ranging from 7 to 42oC. The parasitism of propagules of S. minor and S. sclerotiorum by the isolate IBLF 914, as well as the number of lettuce seedlings surviving drop, was evaluated at 12, 17, 22, 27 and 32oC, in gerboxes containing substrate. S. minor and S. sclerotiorum showed mycelial growth at temperatures ranging from 7 to 27°C, but no growth occurred at 32 °C, and both pathogens had greater mycelial growth at 22°C. The isolates of Trichoderma grew at temperatures ranging from 12 to 37°C, with maximum growth at 27°C. The isolate IBLF 914 had mycoparasitism and reduced the disease in lettuce seedlings at temperatures ranging from 22 to 32°C. Since lettuce drop occurs when mild temperatures and high humidity prevail and the antagonist was more effective at higher temperatures, it is recommended that Trichoderma is applied in lettuce fields in Brazil also during warmer months of the year to reduce the inoculum remaining in the soil before planting the winter crop, which is more affected by the disease.

Selection of Trichoderma isolates for biological control of Sclerotinia minor and S. sclerotiorum in lettuce

ABSTRACT Lettuce drop is one of the most important and difficult-to-control diseases affecting lettuce in Brazil and worldwide. This study was carried out to select Trichoderma isolates antagonistic to Sclerotinia minor and S. sclerotiorum, aiming to develop biological control for this pathosystem in Brazil. Thirty-one Trichoderma isolates were obtained with the use of baits and were tested under laboratory conditions for their ability to control S. minor and S. sclerotiorum in seedlings of lettuce cultivar Tainá cultured in Petri dishes containing water-agar medium. Subsequently, four isolates effective for control and showing high sporulation under laboratory conditions were evaluated in greenhouse in two experiments carried out with both pathogens in lettuce seedlings of the same cultivar. Twenty-two isolates showed ability to control S. minor and S. sclerotiorum in the in vitro experiments. The isolates tested under greenhouse conditions, identified as T. asperellum (IBLF 897, IBLF 904 and IBLF 914) and T. asperelloides (IBLF 908), reduced lettuce drop of seedlings caused by both pathogens but were more effective against S. minor. Biological control is a promising technology for the management of lettuce drop, especially because S. minor is the predominant species in infested lettuce fields in Brazil.