Jürgen Kroschel

A Temperature-Based Simulation Model for the Potato Tuberworm, Phthorimaea operculella Zeller (Lepidoptera; Gelechiidae)

Abstract The potato tuberworm, Phthorimaea operculella Zeller, is a serious pest of the potato, Solanum tuberosum L., in subtropical and tropical production systems around the world. Knowledge of the temperature-dependent population growth potential is crucial for understanding population dynamics and implementing pest control strategies in different agro-ecological zones. The development, mortality of immature life stages, and reproduction of P. operculella were studied at constant temperatures ranging from 10 to 32°C. The theoretical developmental thresholds were 11, 13.5, and 11.8°C, and required incubation times were 65.3, 165.1, and 107.6 degree-days (DD) for the egg, larval, and pupal stages, respectively. The nonlinear shape of the temperature–development curve at low temperatures was well described by the modified four-parameter Sharpe & DeMichele model. The log-normal function was fitted to the normalized cumulative frequency distributions of developmental times for each life stage. Temperature effects on immature mortality were described by polynomial regressions. The optimal temperature for survival was within the range of 20–30°C. Temperature effects on adult senescence were described by the modified Sharpe & DeMichele model. A polynomial function was fitted to total fecundity and temperature. Fecundity was highest around 21°C. Age-related cumulative proportions of fecundity were well described by a Gamma function. Most eggs were laid within the first quarter of the female life span. The established functions were used to build a P. operculella population model, and life table parameters were simulated over a range of temperatures. Calculations gave good predictions when compared with published data. Populations increase within a temperature range of 10–35°C, with an optimum at 28–30°C.

An improved method to determine the biological activity (LC50) of the granulovirus PoGV in its host Phthorimaea operculella

Granulovirus (PoGV) is a promising candidate to substitute for chemical insecticides in integrated pest management (IPM) of the potato tuber moth, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae). A procedure to assess pathogenicity (LC50) and its standardization is crucial for research into the use of pathogens as biocontrol agents. In the present study, an egg‐dip bioassay method was developed and its precision tested in a series of six bioassays over a time period of 18 months. Bioassays were carried out at 25 °C incubation temperature. The probit model as a statistical analysis method for the interpretation of concentration responses proved better than the exponential model. LC50 values ranged from 2.3 × 106 to 107 granules ml−1 with a mean of 5 × 106 granules ml−1 and regression slopes varied between 0.73 and 1.05 with a mean of 0.84. LC50 values and slopes displayed no significant differences according to their 95% confidence limits. The rate of harvested infected larvae showed a linear increase with increasing concentration on a log‐log scale. The mean natural mortality was 15% (SE ± 1.8%) and can be considered as a main factor producing variability among bioassays. The advantages of this bioassay method and its application in PoGV research are discussed.

Chlamydospores of Fusarium Oxysporum Schlecht f.sp. orthoceras (Appel & Wollenw.) Bilai as Inoculum for Wheat-flour—Kaolin Granules to be Used for the Biological Control of Orobanche cumana Wallr.

The formation of chlamydospores of Fusarium oxysporum Schlecht. f.sp. orthoceras (Appel & Wollenw.) Bilai, a potential mycoherbicide for the parasitic weed Orobanche cumana Wallr., was optimised regarding the composition of the liquid growth medium. After 16 days of incubation, highest chlamydospore counts (approximately 2 × 107ml−1) were determined in an aqueous solution amended with 1.5% (w/v) finely ground sorghum stover and 30% (v/v) wheat-based stillage, a liquid by-product of ethanol production. The produced fungal biomass retained viability after being air-dried and stored. Granules made from wheat-flour and kaolin (Pesta) containing microconidia or air-dried chlamydospore-rich biomass were prepared and showed high efficacy in controlling O. cumana in the greenhouse. No differences in efficacy were observed between the different types of inoculum used.

Fungi of Orobanche aegyptiaca in Nepal with Potential as Biological Control Agents

A survey of pathogens of the root-parasitic weed Orobanche aegyptiaca Pers. was carried out in Nepal. More than 70% of the fungal strains isolated from infected plants belonged to the genus Fusarium. Other fungi isolated were Acremonium fusidioides, Alternaria alternata, Cladosporium cladosporioides, Epicoccum nigrum, Mortierella alpina, Papulaspora sp., Phoma spp., Sordaria fimicola , Rhizoctonia sp., Trichoderma spp. and Trichothecium roseum.

Influence of agricultural by-products in liquid culture on chlamydospore production by the potential mycoherbicide Fusarium oxysporum Foxy 2.

Economically feasible inoculum mass production methods are required for successful application of Fusarium oxysporum Foxy 2 as a potential mycoherbicide. Therefore, different substrates (agricultural by-products) and the factors that influence the production of spores, especially chlamydospores, of Foxy 2 were investigated in liquid cultures. The substrates tested were cotton seed cake, maize stover, wheat and triticale stillage. The presence of plant fibers in the medium of unfiltered cotton seed cake (2.5%, w/v) significantly enhanced chlamydospore, micro- and macroconidia production by 150, 185, and 300%, respectively, compared to the filtered (fiber-free) medium. Regardless of the type of substrate tested, Foxy 2 was able to produce abundant chlamydospores (0.14-2.7×107 mL−1) in all growth media. Generally, increasing the concentrations of cotton seed cake and maize stover in the medium significantly increased chlamydospore formation; however, this was not the case for wheat-based stillage. To optimize conditions required for chlamydospore production of Foxy 2, the effect of near ultra-violet (NUV) light, substrate combinations (synergism), level of agitation, incubation time and their interaction were studied. A liquid culture of 2.5% (w/v) filtered cotton seed cake, exposed to continuous NUV for 15 days, doubled the yield of chlamydospores (4.7×106 mL−1) and macroconidia (5×105 mL−1), and increased microconidial production by one-third (1×108 mL−1) compared to natural light. An apparent synergistic effect of substrate combination was observed, since the addition of 20% (v/v) of either wheat or triticale-based stillage to maize stover medium (1%, w/v) increased the number of chlamydospores produced by 16 or 12 times, respectively, compared to maize stover alone (1.4×106 mL−1), and 2 times more than either of the stillages. A significantly positive effect between a high level of agitation tested during incubation and chlamydospore production of Foxy 2 was recorded. All in all, substantial chlamydospore production of Foxy 2 (4.3×107 mL−1) was successfully achieved within 12 days of incubation in a single-step liquid fermentation through the combination of 0.5% (w/v) maize stover plus 20% (v/v) wheat-based stillage and a high level of agitation (200 rpm).

Biological Control of Root Parasitic Weeds with Plant Pathogens

Root parasitic weeds are nutrition specialists. Using special organs, the haustoria, they penetrate into the vessels of roots of host plants in order to supply themselves with water and nutrients. They can be either facultative or obligate, obligate parasitic weeds being considered as one of the most serious constraints to food production in many parts of the world. Obligate parasites are either hemi-parasites with chlorophyll, like species of the genus Striga (Scrophulariaceae), witchweeds, or holo-parasites without chlorophyll such as species of the genus Orobanche (Orobanchaceae), broomrapes. Forty Striga species are reported world-wide: 33 in Africa, 7 in Asia and 4 endemic in Australia. Eleven species are parasites on agricultural crops. The genus Orobanche has more than 100 species but only 7 are considered as economically significant parasitic weeds (Parker and Riches, 1993; Raynal-Roques, 1996).