Target and non-target site mechanisms of fungicide resistance and their implications for the management of crop pathogens

Fungicides are indispensable for high-quality crops, but the rapid emergence and evolution of fungicide resistance have become the most important issues in modern agriculture. Hence, the sustainability and profitability of agricultural production have been challenged due to the limited number of fungicide chemical classes. Resistance to site-specific fungicides has principally been linked to target and non-target site mechanisms. These mechanisms change the structure or expression level, affecting fungicide efficacy and resulting in different and varying resistance levels. This review provides background information about fungicide resistance mechanisms and their implications for developing anti-resistance strategies in plant pathogens. Here, our purpose was to review changes at the target and non-target sites of quinone outside inhibitor (QoI) fungicides, methyl-benzimidazole carbamate (MBC) fungicides, demethylation inhibitor (DMI) fungicides, and succinate dehydrogenase inhibitor (SDHI) fungicides and to evaluate if they may also be associated with a fitness cost on crop pathogen populations. The current knowledge suggests that understanding fungicide resistance mechanisms can facilitate resistance monitoring and assist in developing anti-resistance strategies and new fungicide molecules to help solve this issue. © 2023 Society of Chemical Industry.     

Genetic Variation for Maize Epicuticular Wax Response to Drought Stress at Flowering

Drought stress is thought to promote epicuticular wax accumulation on maize leaves, which reduces plant water loss. We evaluated 62 maize inbred lines and their hybrid testcross progeny for epicuticular wax accumulation on flag leaves at flowering under full and limited irrigation regimes. Extracted wax was measured as a percentage of wax weight to leaf weight (WLW) and leaf area (WLA). Eleven genotypes had above average WLW as both inbred lines and hybrid testcrosses. Thirteen genotypes had above average WLA as either inbred lines or hybrid testcrosses. The drought treatment did not significantly alter WLW or WLA. Heritability of WLW was 0.17 (inbred lines) and 0.58 (hybrid testcrosses). Heritability of WLA was 0.41 (inbred lines) and 0.59 (hybrid testcrosses), suggesting it is a better trait than WLW for epicuticular wax screening. Correlations (r) between inbred lines and their testcross progeny were 0.44 and 0.18, for WLW and WLA, respectively. Heritability of grain weight per ear and plot yield was highest in hybrid testcrosses, with no correlation between inbred and hybrid germplasm. It is not warranted to evaluate epicuticular wax accumulation as the sole drought tolerance mechanism. However, it may be a good secondary trait to observe in relation to grain yield production in hybrids tested under water‐limiting conditions.     

Segregation ratios and growth rate in inactive ovine metallothionein 1a-ovine growth hormone transgenic mice

Objectives were to determine whether the oMt1a-oGH transgene shows normal Mendelian segregation and whether oMt1a-oGH mice exhibit normal growth without the zinc supplementation required to increase plasma oGH levels and stimulate growth. Transgenic mice were reciprocally backcrossed for four generations to high growth and control lines to form lines GM and GR, respectively. In the fifth generation, hemizygous transgenic mice (T/-) were crossed within each line. Pooled across backcross generations, there was a deficit (P < 0.001) of T/- progeny in lines GM (31.6%) and GR (22.2%) compared with expected (50%). In the T/- x T/- cross, the combined percentage of homozygous (T/T) and hemizygous transgenic mice was less (P < 0.001) than expected (75%) in both GM (44.2%) and GR (38.5%). Backcross T/- mice had lower (P < 0.05) 3-wk BW and lower (P < 0.001) 6-wk BW and 3- to 6-wk postweaning gains than nontransgenic mice. Similar genotypic differences were found in the T/- x T/- cross. No significant growth differences were found between T/T and T/- progeny. Using segregation ratios from the T/- x T/- mating, the relative fitness estimates of T/T, T/-, and -/- (nontransgenic) mice were 0.345, 0.223, and 1.0, respectively, in line GM and 0.218, 0.205, and 1.0 in line GR. Fitness estimates in the back-cross for T/- and -/- were 0.463 and 1.0 in line GM and 0.285 and 1.0 in line GR. Abnormal segregation ratios may be due to germline mosaicism or reduced fitness due to differential embryo survival. Reduced growth of oMt1a-oGH transgenic mice when the transgene is switched off suggests a subtle developmental abnormality, which may contribute to a reduction in fitness.     

The effects of bovine respiratory syncytial on normal ovine lymphocyte responses to mitogens or antigens in vitro

In the present study peripheral blod mononuclear cells (MNC) obtained from normal uninfected lambs were used to study the possible effects of bovine respiratory syncytial virus (BRSV) on lymphocyte responses to the mitogens, phytohaemagglutinin (PHA), concanavalin A (Con A) and pokeweed mitogen (PWM) in vitro. Live BRSV had a depressive effect on the proliferative responses of normal MNC to PHA, Con A and PWM. Inactivated BRSV and a commercial preparation of prostaglandin E2 were also found to depress the proliferative responses of normal ovine MNC to PHA but recombinant tumour necrosis factor-alpha (TNF-alpha) had no such effect. Serum samples obtained from BRSV-infected lambs contained substances inhibitory to PHA-driven lymphocyte blastogenesis. Memory blastogenic responses to border disease virus (BDV) of lymyphocytes obtained from lambs previously primed with BDV were significantly reduced when lymphocytes were exposed to infectious BRSV.     

Transendoscopic biopsy of the horse's airway mucosa

Transtracheal wash and bronchoalveolar lavage are diagnostic techniques that have been adopted from human medicine for monitoring inflammatory changes in the airway of the horse. Transendoscopic biopsy has also proven to be a valuable tool for obtaining samples of the airway mucosa in human patients. A transendoscopic technique was developed in this study for obtaining a respiratory mucosal biopsy from standing, sedated horses. Six normal adult horses were sampled at eight-week intervals for a total of three sample periods. Horses were monitored for adverse effects of the technique and none were noted. Sample sites were completely healed after eight weeks with no gross or histologic abnormalities. Biopsy samples were 3 to 4 millimeters in diameter, and 17 of 18 samples provided interpretable histological sections. Methods for handling, staining and evaluating tissue were also developed. The results of this study demonstrated that airway mucosal biopsy is a safe, repeatable technique that can be performed in the sedated, standing horse.     

Factors influencing the duration of isometamidium chloride (Samorin) prophylaxis against experimental challenge with metacyclic forms of Trypanosoma congolense

The duration of a single isometamidium chloride (Samorin) prophylactic treatment against Trypanosoma congolense ILNat. 3.1 and T. congolense IL 285 was examined in 24 Boran steers with regard to (1) the dose of drug, (2) the level of metacyclic challenge and (3) the influence of infection with an unrelated serodeme at the time of treatment. The cattle were repeatedly challenged at monthly intervals between 2 and 7 months following treatment, either by five infected Glossina morsitans centralis or by intradermal inoculation of 5 X 10(3) or 5 X 10(5) in vitro-derived metacyclic trypanosomes. A dose of 1 mg kg-1 afforded complete protection for 4 months and 0.5 mg kg-1 for 3 months against the two T. congolense serodemes examined, irrespective of the method or weight of challenge. In another group of cattle, which had an established infection at the time of treatment, the duration of chemoprophylaxis against an unrelated serodeme was the same as the other groups which had no previous experience of trypanosome infection. Antibodies to metacyclics did not appear in any of the cattle as long as the chemoprophylaxis was effective. An exception to this was the group challenged with 5 X 10(5) in vitro-derived metacyclic parasites, in which low antibody titres were detected. In all cases these proved to be non-protective. It was concluded that, under the experimental conditions employed, (1) there was a direct relationship between drug dosage and the duration of chemoprophylaxis, (2) the weight of metacyclic challenge did not affect the duration of chemoprophylaxis and (3) when used to treat an existing infection, isometamidium chloride exerted the same degree of chemoprophylactic activity.     

INTRATHORACIC SURGERY IN LARGE ANIMALS

The techniques for successful intrathoracic surgery are similar in all species. Basic surgical and anesthetic principles apply in large animals, but special consideration must be given to mechanical problems.