- Author: Tunyalee Martin
- Author: Chris Laning
Identifying nontarget crop and ornamental plant damage from herbicides has become much easier with the launch of a new online photo repository by the Statewide IPM Program, University of California Division of Agriculture and Natural Resources.
Herbicides applied to manage weeds may move from the site where it was applied in the air or by attaching to soil particles and traveling as herbicide-contaminated soil. When an herbicide contacts a nontarget plant, a plant it was not intended to contact, it can cause slight to serious injury. Herbicide injury also occurs when the sprayer is not properly cleaned after a previous herbicide application. Herbicide residue can be found in the spray tank, spray lines, pumps, filters and nozzles so a sprayer must be thoroughly cleaned after an application. Dry herbicide particles can be redissolved months later and cause herbicide damage to plants. Economic damage includes reduced yield, poor fruit quality, distorted ornamental or nursery plants, and occasionally plant death.
Accurately diagnosing plants that may have herbicide injuries is difficult. In many cases, herbicide symptoms look very similar to symptoms caused by diseases, nutrient deficiencies, environmental stress and soil compaction. Plant disease symptoms such as mottled foliage, brown spots or stem death and plant pests such as insects or nematodes cause foliage to yellow and reduce plant growth similar to herbicide injury.
Dr. Kassim Al-Khatib, weed science professor at UC Davis and director of the UC Statewide Integrated Pest Management Program (UC IPM), has gathered nearly a thousand photos of herbicide-damaged plants, drawn from his own and others' research. The images are cataloged to show damage that can occur from 81 herbicides in more than 14 specific herbicide modes of action, applied in the field to demonstrate the symptoms or when known herbicide spray has drifted onto the plant.
Each image is characterized with the name of the plant, mode of action of the herbicide, and notes the specific symptoms of damage. Together these photos provide a comprehensive archive of damage to over 120 different crops and ornamental plants by known herbicides, which users can easily compare with what they see in the field.
Also included in the repository is information about the modes of action of various herbicides and an index of example herbicide trade names and active ingredients. Users can learn how unintended injury from herbicide occurs from misapplication and carryover from previous crops in addition to drift and herbicide-contaminated tanks.
The repository can be found at http://herbicidesymptoms.ipm.ucanr.edu. Increased knowledge about what causes herbicide damage and how it occurs can lead to fewer cases of herbicide injury occurring through drift or herbicide-contaminated tanks. Using the repository can increase the skill to correctly identify plant damage. Correctly identifying damage as herbicide injury and not from a plant pest or nutrient deficiency can prevent unnecessary applications of pesticides or fertilizers. Fewer applications can lessen the risk of harm of pesticides and fertilizers to people and the environment.
Entomopathogenic fungus Beauveria bassiana is known to endophytically colonize various plants and provide protection against arthropod pests. Information of such endophytic interaction of another entomopathogenic fungus Metarhizium brunneum (=M. anisopliae) is limited.
A greenhouse study was conducted in 2010 to evaluate the endophytic potential of B. bassiana (commercial isolate GHA and a California isolate SfBb1) and M. brunneum (commercial isolate F52 and a California isolate GmMa1). Strawberry plants were grown in pots and fungal inocula were applied to the potting medium, vermiculite. When roots and aerial parts were periodically sampled, surface sterilized, and plated on selective media, B. bassiana grew from roots, petioles, pedicels, leaf lamina, sepals, and calyxes whereas M. brunneum was never detected from those tissues. It was initially thought that M. brunneum did not colonize strawberry plants.
However, there was an accidental infestation of twospotted spider mite, Tetranychus urticae on strawberry plants meant for another repetition of the endophyte study with M. brunneum isolates. Among those plants, 32 were treated with M. brunneum isolates and 20 were untreated control plants. Treatments were administered by applying 100 ml of conidial suspension at 1X10^10 conidia/ml concentration around the base of each potted plant. Each isolate had 16 strawberry plants. Mite counts were not taken as the plants were initially intended for endophyte evaluation and leaves could not be destructively sampled. But the proportion of plants damaged by mite infestations were recorded 10 and 14 days after fungal inoculation.
Plants treated with M. brunneum isolates appeared to withstand spider mite infestations better than untreated controls. Since M. brunneum could not be detected in the plant tissue in the previous attempt, it was not clear at that time how the fungus helped strawberry plants to withstand mite damage.
A recent study using scanning electronic microcopy showed that M. brunneum endophytically colonized cowpea plants. It is possible that M. brunneum colonized strawberry plants, but could not be detected using selective medium technique. Another study demonstrated that B. bassiana and M. brunneum promoted the growth of cabbage plants and improved the biomass. In the current study, M. brunneum probably improved the moisture absorption in strawberry plants through mycorrhizal interaction and helped withstand the spider mite infestations which are usually worse in plants under water stress. Fungal toxins in strawberry plants might have also impacted spider mites in a manner similar to the effect of endophytic B. bassiana on green peach aphid, Myzus persicae, in a different study. Observations from the current study indicate the potential of M. brunneum as an endophyte in protecting plants from arthropod damage. Additional studies are required to further investigate this interaction.
Acknowledgment: Thanks to Dale Spurgeon, USDA-ARS for providing laboratory and greenhouse resources for this study.
Dara, S. K. and S. R. Dara. 2015. Entomopathogenic fungus Beauveria bassiana endophytically colonizes strawberry plants. UCANR eNewsletter Strawberries and Vegetables, February 17, 2015.
Dara, S. K., S. S. Dara, and S. S. Dara. 2014. Entomopathogenic fungi as plant growth enhancers. 47th Annual Meeting of the Society for Invertebrate Pathology and International Congress on Invertebrate Pathology and Microbial Control, August 3-7, Mainz, Germany, pp. 103-104.
Golo, P. S., W. Arruda, F. R. S. Paixão, F. M. Alves, E.K.K. Fernandes, D. W. Roberts, and V.R.E.P. Bittencourt. 2014. Interactions between cowpea plants vs. Metarhizium spp. entomopathogenic fungi. 47th Annual Meeting of the Society for Invertebrate Pathology and International Congress on Invertebrate Pathology and Microbial Control, August 3-7, Mainz, Germany, pp. 104.
Vega, F. E., F. Posada, M. C. Aime, M. Pava-Ripoll, F. Infante, and S. A. Rehner. 2008. Entomopathogenic fungal endophytes. Biol. Con. 46:72-82.
Entomopathogen Beauveria bassiana is a soilborne fungus which is commercially available for pest management in organic and conventional agriculture. Although numerous studies demonstrated the interaction of B. bassiana with various arthropod hosts as a pathogen, information on its interaction with plants is limited. Some recent studies investigated the endophytic (growing inside the plant) interaction of entomopathogenic fungi with different species of plants in an effort to understand the impact on arthropods feeding on the plants and antagonistic effect on plant pathogens. When an entomopathogen is present in a plant as an endophyte, it may not cause infection in its arthropod host, but can affect its growth and development through (fungal) toxins. This interaction could be utilized to improve pest control efficacy and improve plant health.
To evaluate the ability of B. bassiana to endophytically colonize strawberry plants, two greenhouse studies were conducted in 2010 using a commercial isolate (GHA) and a California isolate (SfBb1). The first study examined three methods of inoculating strawberry plants where dry conidia of B. bassiana were mixed with potting medium (1X10^7 conidia/gram of vermiculite), strawberry roots were dipped in conidial suspension (1X10^7 conidia/ml) prior to planting, or 100 ml of conidial suspension (1X10^7 conidia/ml) was applied at the base the plant. Care was taken to prevent the contamination of aerial parts of plants with fungal inoculation. Each treatment had four potted plants and a set of untreated plants was used as control. Root, petiole or pedicel, and leaf lamina or sepal or calyx samples were collected 1, 3, and 6 weeks after inoculation to test for the presence of B. bassiana. Plant material was plated a selective culture medium after surface sterilization with bleach solution. Fungal growth from the plant tissue was microscopically examined and identified. Beauveria bassiana emerged from all plant tissues – roots underground to all aboveground parts – throughout the observation period. Among the inoculation methods, root dip and application of conidial suspension caused 52 and 44% of tissue colonization, respectively, followed by 4% colonization from mixing dry conidia.
The second study was conducted to evaluate colonization of B. bassiana at 1X10^9, 1X10^10, and 1X10^11 conidia/ml concentrations. Application of conidial suspension was chosen as it was the easiest means of inoculation and also practical to administer through drip irrigation system in the commercial fields. Treatments were administered by applying 100 ml of respective concentrations of conidial suspensions around the plant base. Plant tissues were sampled 1, 3, 6, and 9 weeks after inoculation using the abovementioned protocol.
Data were subjected to statistical analyses and significant means were separated using Tukey's HSD test.
Both commercial and California isolates colonized all sampled strawberry plant parts for up to 9 weeks after inoculation (Fig. 1). Due to the limited number of plants used in the study, sampling could not be continued beyond 9 weeks.
Fig. 1. Proportion of various plant parts endophytically colonized by commercial (GHA) and California (SfBb1) isolates of B. bassiana at 1, 3, 6, and 9 weeks after inoculation (WAI).
When concentrations were compared, fungal colonization of plants was the highest at 1X10^11 conidia/ml only for the commercial isolate (Table 1). There was no significant difference among conidial concentrations for the California isolate. In general, colonization was first noticed in roots and then the fungus moved up to the aerial parts. This trend was more evident for the commercial isolate with significant differences at 1X10^10 conidia/ml. Although not significant, it appeared that the commercial colonized strawberry plants more than the California isolate.
Table 1. Proportion of different strawberry plant parts endophytically colonized by commercial (GHA) and California (SfBb1) isolates of B. bassiana at various conidial concentrations.
*Average colonization of all plant parts for GHA isolate was significantly different at different concentrations P=0.03). Means followed by the same lowercase letter or no letter in the column were not significantly different.
**Colonization was significantly different among different plant parts at 1010 conidia/ml for GHA (P=0.01). Means followed by the same uppercase letter or no letter in the row were not significantly different.
These are the first studies to demonstrate that B. bassiana endophytically colonizes strawberry plants. The impact of endophytic B. bassiana on arthropod pests attacking strawberry plants was investigated in other studies.
Acknowledgment: Thanks to Dale Spurgeon, USDA-ARS for providing laboratory and greenhouse resources for these studies./span>
- Author: Surendra K. Dara
Understanding the behavior of a pest is very important in developing appropriate control strategies. Information on feeding, host searching, migratory, and reproductive behavior of the invasive Bagrada bug is very limited in published literature. Since Bagrada bug is a fairly new pest in the United States, there is a lot to learn and understand about this pest. Here is a summary of observations about its feeding and reproductive behavior.
Bagrada bugs are primarily attracted to cruciferous crops. However, the number of host species this pest feeds on or passing through is increasing as it spreads to different parts of California. In addition to various wild and cultivated cruciferous plants, Bagrada bugs have been reported to cause damage to carrots, corn, peppers, potatoes, tomatoes, and sunflower. In an earlier choice study where different host plants were offered, neither adults nor nymphs chose tomatoes when alyssum, broccoli, green bean, and wild mustard were among the choices (Dara and Dara, 2013). However, feedback from some growers this year indicated feeding damage to tomatoes (Dara 2014). Although damage was not confirmed, some growers and homeowners reported finding Bagrada bugs on citrus, fig, grape, and strawberry.
Condition of the plants
During a visit to a home garden a couple of years ago, I noticed several Bagrada bugs on dried branches of wild mustard, although different cruciferous vegetable plants were in the proximity. Considering the ability of Bagrada bugs to move around easily, this observation suggests their preference for certain plant conditions. In a recent visit to a 4-week old broccoli field, Bagrada bugs and their damage was noticed only on small and weak plants. Heavy winds a few weeks earlier affected some plants which were significantly smaller than the rest of the plants and were breaking at the base with a slight touch. Similarly, in my lab colony, several bugs are frequently seen on relatively drier plant material although fresh plant material is also present. All these observations suggest that the concentration of plant juices could be influencing Bagrada bugs choice within a specific host. This could mean that maintaining good health of the plants through optimal irrigation and nutrient management is important to avoid weaker plants that could attract Bagrada bugs.
Bagrada bugs are known to hide in the cracks of top soil during cooler parts of the day. Even during warmer parts of the day, some bugs were seen in the soil. This behavior could be exploited by the use of entomopathogens such as Beauveria bassiana and Metarhizium brunneum, which are soilborne fungi. Applied through drip irrigation or as a foliar spray, these fungi can be introduced into the Bagrada bug habitat. Natural behavior of the Bagrada bug to dwell in the soil increases its chances of exposure to fungal inoculum. Although solar radiation might inactivate fungal spores on exposed plant surfaces, being soilborne fungi, these pathogens can persist in the soil for longer periods. Preliminary laboratory assays already demonstrated the potential of these fungal pathogens (Dara 2013).
Based on laboratory observations, Taylor and Bundy (2013) indicated that Bagrada bugs preferred dry soil compared to moist soil to deposit eggs. While this might be the case when Bagrada bugs feed on wild hosts in uncultivated areas, cultivated crops are frequently irrigated and how the soil moisture influences their oviposition behavior in the field conditions is not clear. Earlier literature indicated that eggs are also deposited on various plant parts. Whether eggs are deposited on the plant or in the soil, entomopathogenic fungi could still be important to cause mortality in newly emerged nymphs that might walk on fungal inoculum. If Bagrada bugs overwinter as eggs in the soil, cultivation can be a tool to reduce their numbers. Some entomopathogenic fungi cause egg mortality in addition to infecting mobile stages.
Nature and Numbers
Bagrada bugs have a wide host range and some of their preferred hosts are spread across large areas as wild plants. When these plants dry out, they migrate to crop plants in significant numbers. This is probably why control with pesticide applications alone or using trap crops can be challenging. Some community and home gardeners who tried to use trap crops or traps with alyssum, were able to find large numbers in those crops or traps, but even larger numbers continued to move to crop plants. For a pest like Bagrada bug, exploiting natural enemies appears to be a crucial management tool. Arrangements for foreign exploration of natural enemies are underway.
Dara, S. K. 2013. Bagrada bug update: bioassays and a short video.
Dara, S. K. 2014. Current status of the invasive Bagrada bug in California: geographic distribution and affected host plants.
Dara, S. K. and S. S. Dara. 2013. Bagrada bug host preference: crucifers and green beans.
Taylor, M. and C. S. Bundy. 2013. The life history and seasonal dynamics of Bagrada hilaris in New Mexico. Annual meetings of the Entomological Society of America, Austin, TX.
Beneficial fungi such as Beauveria bassiana are pathogenic to insect and mite pests and are commercially available for use in organic and conventional farming. Field studies conducted on commercial strawberry farms with B. bassiana and another entomopathogenic fungus, Metarhizium brunneum show the importance of these microbial pesticides in pest management on conventional farms (Dara 2013, 2014, and unpublished). These studies can make a significant contribution to IPM practices by reducing chemical pesticide use without compromising the pest management efficiency.
In a cropping system where fungicides are frequently applied for managing various foliar diseases such as powdery mildew (caused by Podosphaera aphanis) and botrytis fruit rot (caused by Botrytis cinerea), the fate of a beneficial entomopathogenic fungus is always an important question. Evaluating the compatibility of various fungicides commonly used in strawberries with B. bassiana is necessary to understand the fungicide and beneficial fungus interactions. A series of studies were conducted to address this issue and to explore opportunities to evaluate their compatibility.
In 2012, six bioassays were conducted using fungicides Captan, Elevate, Microthiol Disperss, Pristine Quintec, Rally, and Switch and an organic formulation of B. bassiana (Mycotrol-O) (Dara and Dara, 2013). Mortality and/or infection caused in mealworm (Tenebrio molitor) larvae exposed to surfaces treated with B. bassiana and fungicide was used as a measure of compatibility between the fungicides and the beneficial fungus. Except for Elevate and Quintec, all other fungicides showed moderate to high level of inhibitory effect on the fungus. A follow up study with Pristine showed that increasing the application interval to 1 or 4 days improved the compatibility and resulted in 100% mortality of the mealworms from B. bassiana treatment. Another study was conducted where B. bassiana (BotaniGard EX) was applied 0 to 6 days after fungicides Pristine, Merivon, and Switch were applied (Dara et al. 2014). Switch seemed to have a higher negative impact on B. bassiana than Pristine and Merivon, in general, but the increase or decrease in mealworm mortality with increasing time interval between the fungicides and fungus was variable. Although these two studies indicated that increasing time interval could influence the compatibility of fungicides and B. bassiana,they were conducted only once and warranted additional replicated studies.
A new study was conducted from June to August, 2014 where eight fungicides that had different modes of action were applied at 0 to 6 day intervals to evaluate their impact on mealworm mortality caused by B. bassiana.
Positive control with BotaniGard ES® (B. bassiana)
BotaniGard ES applied 0,1, 2…6 days after treating with Captan.
BotaniGard ES applied 0,1, 2…6 days after treating with Pristine.
BotaniGard ES applied 0,1, 2…6 days after treating with Merivon.
BotaniGard ES applied 0,1, 2…6 days after treating with Microthiol Disperss.
BotaniGard ES applied 0,1, 2…6 days after treating with Rally.
BotaniGard ES applied 0,1, 2…6 days after treating with Rovral.
BotaniGard applied 0,1, 2…6 days after treating with Switch.
BotaniGard ES applied 0,1, 2…6 days after treating with Thiram.
Captan alone applied 0, 1, 2…6 days prior to the exposure.
Pristine alone applied 0, 1, 2…6 days prior to the exposure.
Merivon alone applied 0, 1, 2…6 days prior to the exposure.
Microthiol Disperss alone applied 0, 1, 2…6 days prior to the exposure.
Rally alone applied 0, 1, 2…6 days prior to the exposure.
Rovral alone applied 0, 1, 2…6 days prior to the exposure.
Switch alone applied 0, 1, 2…6 days prior to the exposure.
Thiram alone applied 0, 1, 2…6 days prior to the exposure.
Including the untreated control, there were a total of 114 treatments in each assay. Each treatment had 10 mealworms that were individually incubated in Plexiglas vials with a piece of carrot after a 24 hour exposure to a paper towel treated with B. bassiana, fungicide, or B. bassiana+fungicide applied at different time intervals. Mortality of the worms was observed daily for 6 days. Treatments of fungicides without B. bassiana were also included to see if they have any influence on the mortality of the worms. These assays were repeated three times using medium-sized mealworms purchased from a commercial supplier.
None of the worms in untreated control died during the study. Except for six dead worms out 560 in fungicide only treatments in the first assay, there did not seem to be any impact of fungicides alone on the mortality of mealworms.
Among the fungicides tested, Captan (Mode of action group M4) and Thiram (Mode of action group M3) are the only ones that showed a significant negative impact on B. bassiana resulting in reduced mealworm mortality (Fig. 1, Table 1). Other fungicides had no or negligible impact on B. bassiana. When the average total mortality of the mealworms among different time intervals between B. bassiana and fungicides was considered, Captan caused about 57% reduction and Thiram caused 43% reduction in the efficacy of B. bassiana. Remaining fungicides caused only 0-2% of reduction in the efficacy of B. bassiana. Both Captan and Thiram are broad spectrum fungicide acting through multi-site contact and differ from others, except for Microthiol Disperss (Mode of action group M2), in their modes of action.
Time interval between B. bassiana and different fungicides did not seem to have any impact on the total mortality of mealworms. Although the total mortality caused by B. bassiana ranged from 30-57% in Captan and 33-77% in Thiram treatments at different time intervals, differences were not statistically significant (P > 0.05).
Fig. 1. Average total mortality of mealworms at different time intervals between B. bassiana and fungicides
Table 1. Total mortality caused by B. bassiana when fungicides were applied at different time intervals.
*Means followed by the same letter within each column are not statistically significant (Tukey's HSD P > 0.05). There was no significant difference in values within each row i.e., no difference in time intervals between B. bassiana and any of the fungicides.
This study shows that several of the fungicides commonly used in strawberries are compatible with B. bassiana. When B. bassiana is considered for pest management, Captan and Thiram should be avoided. Fungus-based microbial pesticides play an important role in conventional agriculture and understanding their interaction with fungicides helps with their effective use in pest management.
Dara, S. 2013. Microbial control as an important component of strawberry IPM. February issue of CAPCA's Adviser magazine, pp 29-32.
Dara, S. 2014. New strawberry IPM studies with chemical, botanical, and microbial solutions. February issue of CAPCA Adviser magazine, pp 34-37.
Dara, S. and S.S.R. Dara. 2013. Compatibility of the entomopathogenic fungus, Beauveria bassiana with some fungicides commonly used in strawberries. Strawberries and Vegetables Newsletter (http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=9626)
Dara, S. S., S.S.R. Dara, and S. Dara. 2014. Optimal time intervals for using insect pathogenic Beauveria bassiana with fungicides. Central Coast Agriculture Highlights (http://cesantabarbara.ucanr.edu/newsletters/Central_Coast_Agriculture_Highlights50500.pdf)