Dose Curves of Disinfestants Applied to Plant Production Surfaces to Control Botrytis cinerea

A Systematic Review and Quantitative Synthesis of the Efficacy of Quaternary Ammonium Compounds in Disinfesting Nonfungal Plant Pathogens

This quantitative review and systematic analysis of the effectiveness of quaternary ammonium compounds (QACs) in disinfesting nonfungal plant pathogens in agricultural and horticultural cropping systems is a complementary follow-up to a previous study that evaluated the efficacy of QACs against fungal plant pathogens. In the present study, a meta-analysis involving 67 studies was conducted to assess the overall efficacy of QACs against plant pathogenic bacteria, oomycetes, and viruses and to identify factors associated with observed differences in product efficacy. Across all studies, QACs resulted in a significant (P < 0.0001) reduction in either disease intensity or propagule viability with a mean Hedges' g ([Formula: see text]) of 1.75, indicating that overall QAC treatments were moderately effective against nonfungal pathogens. Significant differences in product efficacy were observed between organism types (P = 0.0001), with QAC interventions resulting in higher efficacy (P = 0.0002) against oomycetes ([Formula: see text] = 4.20) than against viruses ([Formula: see text] = 1.42) and bacteria ([Formula: see text] = 1.07), which were not different (P = 0.2689) from each other. As a result, bacterium and virus types were combined into a composite set (BacVir). QAC intervention against BacVir resulted in significant differences in efficacy within categorical moderator subgroups for genus (P = 0.0133), target material (P = 0.0001), and QAC product generation (P = 0.0281). QAC intervention against oomycetes resulted in significant differences in efficacy only for genus (P < 0.0001). For the BacVir composite, five random effect (RE) meta-regression models were significant (P = 0.05), where models with dose and time, dose and genus, time and genus, dose and target, and time and target accounted for 62, 61, 52, 83, and 88%, respectively, of the variance in true effect sizes (R2) associated with [Formula: see text]. For oomycetes, three RE meta-regression models were significant (P = 0.05), where models with dose and time, dose and genus, and time and genus accounted for 64, 86, and 90%, respectively, of R2 associated with [Formula: see text]. These results show that while QACs are moderately effective against nonfungal plant pathogens, the observed variability in their efficacy due to dose of active ingredient and contact time of these products can be influenced by organism type, genus within organism type, the target being treated, and the generation of QAC products.

A Systematic Review and Quantitative Synthesis of the Efficacy of Quaternary Ammonium Disinfestants Against Fungal Plant Pathogens

Quaternary ammonium compounds (QACs) have been used as disinfestants in plant production systems since the late 20th century. In studies on the control of fungal pathogens in agricultural and horticultural crop production systems, the efficacy of QAC disinfestants is variable, ranging from very high to ineffective. A systematic review and meta-analysis were performed to establish and understand how pathogen- and application-related factors influenced product efficacy. The meta-analysis was based on 124 studies involving 14 fungal plant pathogen genera, eight target materials, and four generations of QAC products that contained different mixtures of active ingredients. A significant (P < 0.0001) reduction in either disease intensity or propagule viability resulted following disinfestation using QAC products. Hedges' g standardized mean difference (g̅₊) across the studies was 2.16, indicating that QACs, on average, were highly effective against fungal pathogens. Heterogeneity was significant (P < 0.0001), indicating that effect sizes (g) were not representative of a common mean effect size and supported selection of a random effects model. In all, 78.5% of the observed variance consisted of variance in true effects with a high estimate of between-study variability (τ² = 2.15). For fungal genus, subgroup g̅₊ for genera Pseudonectria and Calonectria was significantly (P < 0.0038) higher than for all other genus subgroups, except Fusarium. For target materials, subgroup g̅₊ for solution, cloth, plant, and metal were significantly (P > 0.0071) higher than for inorganic material or wood. For product generation, subgroup g̅₊ for fifth-generation products was significantly (P > 0.0071) higher than for fourth-, third-, and second-generation products. Dose and time accounted for only 8 and 4%, respectively, of the true variance in effect sizes in the regression model dose, time, and dose-time (P = 0.0004). Genus accounted for 40 and 51% of the true variance in effect sizes in the regression models dose and genus (P = 0.0008) and time and genus (P = 0.0007), respectively. Target material accounted for 18 and 19% of the true variance in effect sizes in the regression models dose and target (P = 0.0001) and time and target (P = 0.0001), respectively. QAC product generation accounted for 24 and 21% of the true variance in effect sizes in the regression models dose and QAC generation (P = 0.0034) and time and QAC generation (P = 0.0189), respectively. These results show that the current recommended rates for dose and contact time are generally expected to result in effective disinfestation for commercial QAC products. However, the efficacy against fungal plant pathogens is likely to be influenced by the fungal genus and target being treated and the generation of the QAC product that is used for disinfestation.

Comparative Performance of Sanitizers in Managing Plant-to-Plant Transfer and Postharvest Infection of Calonectria pseudonaviculata and Pseudonectria foliicola on Boxwood

Calonectria pseudonaviculata and Pseudonectria foliicola causing the infamous "boxwood blight" and "Volutella blight," respectively, are a constant threat to the boxwood production and cut boxwood greenery market. Both pathogens cause significant economic loss to all parties (growers, retailer, and customers) in the horticultural chain. The objective of this study was to evaluate efficacy of disinfesting chemicals (quaternary ammonium compound [QAC], peroxy, acid, alcohol, chlorine, and cleaner) in preventing plant-to-plant transfer of C. pseudonaviculata and P. foliicola via cutting tools, as well as reduction of postharvest boxwood blight and Volutella blight disease severity in harvested boxwood greenery. First, an in vitro study was conducted to select products and doses that completely or near-completely inhibited conidial germination of C. pseudonaviculata and P. foliicola. The selected treatments were also tested for their ability to reduce plant-to-plant transfer of C. pseudonaviculata and P. foliicola and manage postharvest boxwood blight and Volutella blight in boxwood cuttings. For the plant-to-plant transfer study, Felco 19 shears were used as a tool for mechanical transfer of fungal conidia. The blades of Felco 19 shears were exposed to a conidial suspension of C. pseudonaviculata or P. foliicola by cutting a 1-cm-diameter cotton roll that had been dipped into a fungal suspension. Disease-free boxwood rooted cuttings (10-cm height) were pruned with the contaminated shears. The Felco 19 shears were equipped with a mounted miniature sprayer connected to a pressurized reservoir of treatment solution that automatically sprayed the blade and plant surface while cutting. The influence of accumulated sap on the shear blade was studied through 1- or 10-cut pruning variable on test plants and screened for the efficacy of treatments. Then, the boxwood rooted cuttings were transplanted and incubated in room conditions (21°C, 60% RH) with 12 h of fluorescent light; data evaluation on disease severity was done weekly for a month. Disease progress (area under disease progress curve [AUDPC]) was calculated. In another study, postharvest dip application treatments were used for the management of postharvest boxwood blight or Volutella blight on boxwood cuttings. The harvested boxwood cuttings were inoculated with a conidial suspension of C. pseudonaviculata or P. foliicola and then dipped into treatment solution 3 days afterward. The treated boxwood cuttings were kept in room conditions, and boxwood blight or Volutella blight disease severity as well as marketability (postharvest shelf life) was assessed every 2 days for 1 week. A significant difference between treatments was observed for reduction of boxwood blight or Volutella blight severity and AUDPC. The treatments [Octyl decyl dimethyl (ODD) + dioctyl dimethyl (DoD) + didecyl dimethyl (DdD) + dimethyl benzyl (DB)] ammonium chloride (AC) (Simple Green D Pro 5), 2-propanol + didecyl dimethyl ammonium chloride (DDAC) (0.12%; KleenGrow), and dimethyl benzyl ammonium chloride (DBAC) + dimethyl ethylbenzyl ammonium chloride (DEAC) (GreenShield) were the most effective in reducing the plant-to-plant transfer of boxwood blight and Volutella blight when pruned with contaminated Felco 19 shears. In addition to the three effective treatments above, acetic acid (2.5%; vinegar), 2-propanol + DDAC (0.06%), sodium hypochlorite (Clorox), and potassium peroxymonosulfate + NaCl (2%; Virkon) were effective in reducing postharvest boxwood blight, whereas DBAC + DBAC (Lysol all-purpose cleaner), ethanol (70% [ethyl alcohol]), and DDAC + DBAC (Simple Green D Pro 3 plus) were effective in reducing Volutella blight disease severity and AUDPC, and they also maintained better quality and longer postharvest shelf life of boxwood cuttings when applied as a dip treatment. The longer postharvest shelf life of boxwood cuttings noted may be attributed to reduced disease severity and AUDPC resulting in healthy boxwood cuttings.

Efficacy of Hypochlorite as a Disinfestant Against Fungal Pathogens in Agricultural and Horticultural Plant Production: A Systematic Review and Meta-Analysis

Hypochlorite is often used as a disinfestant of fungal pathogens in a range of agricultural and horticultural settings. However, reports of its effectiveness are variable across studies and it is unclear what factors could potentially influence the reported estimates of its efficacy. A systematic review and meta-analysis was conducted to assess the efficacy of hypochlorite against fungal pathogens and explore factors that may explain the observed heterogeneity in estimates of efficacy. Standardized mean effect size, Hedges' g, was calculated for each of the 109 selected studies, published from 1972 to 2019, that met the criteria defined for the systematic review. A random-effects model was used to estimate the overall mean effect size ([Formula: see text]) and determine the heterogeneity in g among studies. Hypochlorite resulted in a significant (P < 0.001) reduction in either disease intensity or propagule viability with [Formula: see text]= 2.25, suggesting a large overall effect. However, 95% prediction intervals ranged from -0.18 to 4.68, indicating that hypochlorite could be ineffective against some fungi or when targeting some substrate materials. An estimate of the within-study variability, τ², was 1.48 and the proportion of heterogeneity in g among studies due to true effects was 71.5%. Inclusion of categorical moderator variables in the random effects model showed that hypochlorite treatments were significantly (P < 0.0062) more effective when used to disinfest spores in an aqueous solution ([Formula: see text]= 4.58) than when used on plastic ([Formula: see text]= 2.13), plant ([Formula: see text]= 2.13), and wood ([Formula: see text]= 0.79). Similarly, hypochlorite treatments were significantly (P < 0.0083) more effective in disinfesting fungal propagules of Thielaviopsis spp. ([Formula: see text]= 2.51) than those of Verticillium spp. ([Formula: see text]= 1.21). A meta-regression indicated that the effect of dose (β = -3.54; P = 0.0398) and contact time (β = -0.05; P = 0.0001) on [Formula: see text] were highly significant. Further, [Formula: see text]was significantly affected by the dose × time interaction (β = -0.017; P = 0.0269). In the meta-regression models, dose and time explained 0 and 16% of the variance in true effects, respectively. In meta-regression models with a continuous variable of dose or time, a categorical variable of target or genus and their interaction term, genus and target explained an additional 7 to 19% of the variance in true effects. These results show that although the current recommended dose and contact time for commercial bleach products are expected to result in effective disinfestation, the target material and genera of the fungal pathogen of interest will likely influence their efficacy.

Disinfectant Treatments That Reduce Transmission of Stagonosporopsis citrulli During Cucurbit Grafting

Gummy stem blight can develop in greenhouses on cucurbit seedlings grown as scions and rootstocks for grafting. When diseased seedlings are cut during grafting, Stagonosporopsis spp., the fungal pathogens causing gummy stem blight, may be transferred to healthy seedlings. The objective of this study was to evaluate efficacy of disinfectant treatments to prevent or reduce transmission during cutting and grafting. A blade contaminated with mycelium and spores from a culture of S. citrulli transferred the pathogen to 72%, 73%, and 55% of the second, third, and fourth seedlings, respectively, cut in sequence when 100% of the first seedlings cut were infected. Kleengro (didecyl dimethyl ammonium chloride), Physan 20 (dimethyl benzyl ammonium chloride), and Virkon S (potassium peroxymonosulfate) were ineffective when sprayed onto watermelon seedlings before or after cutting hypocotyls with a contaminated blade. Dipping a contaminated blade in 0.3% sodium hypochlorite, 0.4% Physan 20, or 70% or 95% ethanol before cutting watermelon hypocotyls significantly (P = 0.01) reduced incidence of gummy stem blight compared with water but did not prevent transmission. Soaking a contaminated blade for 3 s in 95% ethanol, 30 s in 0.8% sodium hypochlorite, or flaming the blade after dipping in 95% ethanol, prevented transmission. In a grafting experiment, both the watermelon scion and the interspecific hybrid squash rootstock were cut with contaminated or contaminated and treated blades before grafting. Sterilizing a contaminated blade by flaming significantly (P = 0.01) reduced incidence of gummy stem when compared with a 1-s dip in ethanol, a 1-s dip in Physan 20, or water. Disease incidences in these treatments were 11%, 45%, 100%, and 100%, respectively. Using heat, ethanol, or sodium hypochlorite to disinfest cutting tools may reduce transmission of S. citrulli during cucurbit grafting.

Surprising Results from a Search for Effective Disinfectants for Tobacco mosaic virus-Contaminated Tools

Over 100 years after its discovery, Tobacco mosaic virus (TMV) remains an economically important pathogen for producers of many vegetatively propagated crops including petunias (Petunia × hybrida). To directly address this concern, we have developed a robust system to determine efficacy of disinfectants for treating TMV-contaminated cutting tools using a combination of preliminary screens and replicated trials. Contrary to widely held beliefs, wild-type (wt) TMV and four additional tobamovirus species infected four petunia cultivars without producing obvious viral symptoms. In contrast, a petunia isolate of TMV with 99.0% (nucleotide) and 99.4% (amino acid) coat protein sequence identity to wt TMV produced symptoms on all but one tested cultivar. We also show that TMV transmission can occur up to the twentieth petunia plant cut following a single cutting event on a TMV-infected plant. Although many new products are now available, treatment of TMV-contaminated tools with a 20% (wt/vol) solution of nonfat dry milk (NFDM) plus 0.1% Tween 20 or a 1:10 dilution of household bleach (0.6% sodium hypochlorite), two "old standbys", completely eliminated TMV transmission to petunias. Treatment of contaminated tools with 1% (wt/vol) Virkon S or 20% NFDM also significantly reduced the incidence of infected petunias. Other treatments identified in the preliminary screens are candidates for the second phase of screening that simulates contamination during the process of taking cuttings.