Botrytis can cause devastating losses during production, shipping, and in retail. An Integrated Pest IPM approach is necessary to manage Botrytis successfully, and it is critical for growers to have alternative control methods to use in rotation with fungicides to prevent resistance. Microbial biopesticides are an IPM tool for controlling disease while mitigating fungicide resistance risk. The adoption of biopesticides in production is growing, but their variable performance under commercial conditions is still a challenge. Advances in biopesticide research reveal that they work best when used in combination with other strategies. Several natural compounds have been found to enhance biopesticide efficacy with significant implications for IPM.
In previous AFE-funded research, they found that chitosan reduced symptoms caused by Botrytis on petunia leaves. Chitosan is a natural biostimulant compound derived from chitin that has fungistatic and antimicrobial properties. In this project, they investigated the use of commercially available chitosan products to enhance the efficacy of microbial biopesticides for the control of Botrytis. The objectives were to identify chitosan application rates for use in floriculture to avoid phytotoxicity and evaluate chitosan-biopesticide co-application to suppress Botrytis on petunia.
Since chitosan can cause phytotoxicity, commercial chitosan products were screened to identify application rates that are safe to use and suppress Botrytis on flowers, using petunia as a model crop. Detached flowers were sprayed to glisten with chitosan treatments ranging from 0.1-0.3% chitosan. Treatments included Tidal Grow low molecular weight (MW), Tidal Grow high MW, ARMOUR-Zen 15, and a water control.
Greenhouse experiments were conducted to identify any synergistic effects of commercial chitosan products combined with biopesticides for disease suppression. Four weeks after rooting, petunia plants were sprayed to glisten with the chitosan treatments (Tidal Grow Low MW or ARMOUR-Zen at 0.3%) or with water and allowed to dry. Next, plants were sprayed to glisten with a biopesticide (Cease, Howler, and Botrystop WP) or water.
Twenty-four hours after the treatment application, fifteen leaves per treatment (three from each plant) were collected and inoculated with Botrytis. Additionally, twelve flowers per treatment were removed and inoculated with Botrytis. The leaves and flowers were incubated for 48 hours, and disease severity was measured using a 1-9 disease severity scale (Figure 1).
Figure 1. Rating scale used to assess Botrytis severity on Black Cherry petunia flowers. Percentages represent the flower area showing symptoms. Each flower was assigned a rating (0-9) corresponding to the picture it most clearly resembled.
Results
Products prepared at 0.2% and 0.3% (v/v) chitosan mixed with a nonionic surfactant CapSil (4oz/100gal) sprayed on flowers provided good spray coverage with low phytotoxicity on flowers (Figure 2).
Figure 2. Supertunia Black Cherry flowers 24 hours after being sprayed with three commercial chitosan products at various chitosan concentrations (0.1 – 0.3%) with the addition of the surfactant CapSil (4 oz/100 gal).
As observed in previous experiments, leaves from plants treated with Tidal Grow or ARMOUR-Zen alone had smaller lesions than leaves from plants treated with water. While there was an enhanced effect of the biopesticide + chitosan treatments on reducing lesion size, this was largely due to the reduction in disease caused by the chitosan product. Only leaves from plants treated with the Tidal Grow + Cease combination had less disease than leaves from plants treated with Tidal Grow alone or Cease alone, suggesting a true synergism is occurring between the active ingredient of Cease and chitosan.
Disease suppressive effects and chitosan-biopesticide synergisms were more clearly observed on leaves compared to flowers. This may be because flowers are more susceptible to Botrytis, and inoculated flowers often have high disease severity, regardless of treatment. However, there was a 28% reduction in disease severity in flowers treated with ARMOUR-Zen and Cease compared to flowers just treated with Cease.
The results can be summed up as follows, chitosan products can be applied to petunia plants during flowering at 0.2% and 0.3% (v/v) mixed with a surfactant. Chitosan was more effective at reducing Botrytis compared to the biopesticides, but a synergistic effect was observed on the leaves of plants treated with Tidal Grow + Cease.
The results from this research show that chitosan can be an effective addition to an IPM program for Botrytis management. To see the full research report, click here.