Impact of nutritional conditions on yields,germination rate and shelf-life of Plectosporium alismatis conidia and chlamydospores as potential candidates for the development of a mycoherbicide of weeds in rice crops

The effect of nutritional conditions on spore qualities was investigated in order to select which propagules, conidia or chlamydospores, would be most suitable for mycoherbicide development. Plectosporium alismatis was grown in a liquid basal medium supplemented with glucose and a mineral nitrogen source (sodium nitrate) or an organic nitrogen source (casamino acids). Conidial and chlamydospore yields, germination rate and shelf-life were compared. Two growth models were developed: on one hand, sodium nitrate added as the sole nitrogen source was partially utilised (8%), resulting in poor growth (1.77±0.02 mg mL^?1; 6±1.7×10⁵ conidia mL^?1). Under these conditions, P. alismatis produced dense, melanised-like aggregates that contained chlamydospores (12.4±0.7×10⁴ chlamydospores mL^?1). Germination rates of chlamydospores and conidia produced under these conditions was high (80%). Twenty percent of chlamydospores were able to germinate after 4 months storage at 25°C, while survival of conidia declined rapidly (<2%). When casamino acids were added to the liquid medium as the sole nitrogen source, P. alismatis produced sparser pellets resulting in high dry weights (5.37±0.09 mg mL^?1 and high conidia numbers (9.6±1.5×10⁶ conidia mL^?1), while no chlamydospore were observed. The germination rate of conidia produced in casamino acids was low (33±13%) after 8 h incubation and microcycle conidiation occurred. Five percent of these conidia germinated after 4 months storage. These data indicate that chlamydospores may be suitable for mycoherbicide development, provided further optimisation of yields is achieved.     

Impact of culture age on conidial germination,desiccation and UV tolerance of entomopathogenic fungi

The impact of culture age on conidial yields, germination and tolerance to UV exposure of freshly harvested and dry conidia produced by five entomopathogenic fungal (EPF) isolates was studied. Beauveria bassiana, Metarhizium anisopliae, Lecanicillium lecanii and Lecanicillium muscarium were grown on potato dextrose agar (PDA) medium for 7 or 14 days at 25°C. While the age of cultures had a significant impact on the germination rate of conidia produced by isolates L. lecanii CBS 122.175 and B. bassiana LMSA 1.01.093, other EPF isolates germinated at the same rate regardless of the culture age. When exposed to UV radiation, conidia produced by all isolates germinated at a lower rate compared to the non-irradiated conidia, although this decrease in germination (20–80% decrease) was unaffected by the culture age. Air-drying had only a slight impact on conidial germination (0–60% decrease). Under the conditions of this study, the stability of irradiated conidia produced by M. anisopliae LMSA 1.01.197 and B. bassiana CBS 110.25 was significantly increased when conidia were dried prior to UV exposure. This increase in tolerance to stress of dried conidia might be caused, at least partially, by the low metabolic activity associated with dehydration.     

Enhancing survival and subsequent infectivity of conidia of potential mycoherbistats using UV protectants

Ultraviolet radiation (UV) can reduce the effectiveness of fungi used for biological control; therefore, this study examined the photostabilising effect of water- and oil-soluble UV protectants on conidium germination of Plectosporium alismatis and Colletotrichum orbiculare, pathogens with potential as biocontrol agents, and the ability of conidia of C. orbiculare to cause disease. Formulation in riboflavin (1%), proline (1%), propyl gallate (1%), melanin (0.1%) and ascorbic acid (5%) increased the germination of UVB-exposed conidia of P. alismatis to levels found in the dark control without causing a delay in germination. Formulation in (a) pyridoxin (5%), (b) an nC24 mineral oil (5%), and (c) ECCO 1422 (5% in the mineral oil) also resulted in germination similar to the control but germination was delayed. Protection was provided to conidia of C. orbiculare treated with 1% aqueous solutions of proline and folic acid in vitro. Formulation of conidia of C. orbiculare in a 5% aqueous emulsion of the mineral oil and aqueous solutions of melanin (0.01%), proline and tyrosine (both at 1%) significantly increased anthracnose development above control levels on leaf discs of Xanthium spinosum exposed to UVB dose of 16.7 kJ m^-2. After exposure to natural sunlight at a UVB dose of 2.2 kJ m^-2, anthracnose development was greater on leaf discs inoculated with conidia of C. orbiculare formulated in 1% aqueous solutions of ascorbic acid (1%), proline (1%), tyrosine (1%) and melanin (0.01%), or in 5% aqueous emulsions of a canola-derived oil and the mineral oil than by conidia formulated in water alone. Therefore, a range of compounds can provide conidia with protection from UVB. Of these, propyl gallate and oils similar to the mineral oil are likely to be cost effective. Such formulations can be combined with suitable application times to increase mycoherbisitat efficiency.     

Characterization of aggregates produced by the potential mycoherbistat Plectosporium alismatis in submerged culture: germination,UV-radiation tolerance and infectivity

Plectosporium alismatis, a potential mycoherbistat of Alismatacae spp., has been previously shown to produce aggregates which contain chlamydospores in liquid culture. Weevaluated the impact of medium composition on the formation and composition of aggregates. In shake flasks cultures using 5.74 gL^?1 sodium nitrate, 8.8 gL^?1 malt extract or glucose and 0.1% Tween 80, P. alismatis formed small, uniform (diameter of 75% aggregates <720 µm), dense, melanised aggregates containing 10⁴ conidia and 10³ chlamydospores, these numbers remained unchanged during growth. All 7-day-old aggregates exposed to desiccation or/ and UV-radiation germinated. In bioassays using leaf discs of Alisma plantago-aquatica, P. alismatis aggregates caused necrosis, regardless of whether aggregates had been exposed to desiccation and/or UV-radiation prior to application on leaf discs, whereas other propagules (10³ propagules disc^?1) exposed to drying and UV-radiation stress were unable to cause necrosis. This preliminary research shows the potential of aggregates to be used as part of a formulation of biocontrol agents, provided adequate conditions for optimal aggregate yields are found.     

Storage temperature influences desiccation and ultra violet radiation tolerance of entomopathogenic nematodes

1. The effect of cold (5 °C) and warm (35 °C) storage on desiccation tolerance of cold-adapted Steinernema feltiae, intermediate S. carpocapsae and warm-adapted S. riobrave was evaluated at 5 and 35 °C.