Taking on pests and diseases in cannabis: Vegetative to flowering

See Greenhouse Canada, December 2020

Vegetative to early flowering
During these stages, the threat of crop loss to root diseases remains and biofungicides should be reapplied when young plants are potted up. But now, there is the added challenge of dealing with foliar diseases. 

BCA releases and bioinsecticide sprays should continue. You may be tempted to cut back as pests seem to be 'under control,' but we have seen resurgences in broad mites and thrips when predators have been removed from the program at this stage. Major remediation efforts with biopesticide sprays were then required, and significant crop losses incurred.

Foliar diseases
Cultural practices like pruning and air circulation help keep powdery mildew in check, but the disease invariably shows up. Crop scouting will allow signs of infection to be detected early on, and regular preventative treatments will help contain the disease. 

Potassium bicarbonate products such as MilStop can be used in a preventative or curative manner. K-bicarbonate brings a physical mode of action to bear and will desiccate mildew spores, destroy cell membranes and, by altering the pH at the leaf's surface, create conditions that impede powdery mildew growth. The adjuvants in the formulation can be harsh on soft-bodied biocontrol agents (BCAs), but will not harm predators in sachets. Once spray residues are dry, they are harmless to BCAs. 

Also BCA-friendly is Regalia MAXX biofungicide, a liquid concentrate based on an extract from the giant knotweed. When applied to plants, it induces a resistance response which helps protect it against powdery mildew. Regalia and MilStop are compatible with each other and can be sprayed in rotation. Spray oils, such as Suffoil-X and PureSpray FX, will also help in controlling powdery mildew and are good options closer to harvest.

Pest management
The same list of suspects encountered in propagation are likely to continue through this stage of plant development, notably mites, root aphids, and thrips. Whiteflies are generally not a problem in northern US and Canadian greenhouses. 

The control options for these pests are essentially the same as described before, (i.e. Beauveria-based biopesticides, BCAs - see previous article for details). However, specific use practices and release rates will change as plants are now larger and the substrate volume is greater. A second release of the soil-dwelling predators Stratiolaelaps and Dalotia at potting up should ensure they become established for the remainder of the cropping cycle. 

One addition to the BCA arsenal at this stage is the generalist predator Orius insidiosus. If released into hot spots, it can become established for an extended period from the first treatment or release. However, it is unlikely to persist for several generations in indoor grow facilities under artificial lights and is prone to diapause during the shorter daylight months (Nov to Feb). 

For predatory mites (cucumeris, swirskii, californicus), mini sachets remain the preferred method of delivery into the crop. At this stage of production, however, sachets can now be hung in the canopy of the growing plant. Phytoseiulus persimilis is another valuable player at this stage of crop development and it can be used to clean up two-spotted spider mite (TSSM) hot spots if needed. Again, scouting is an important part of the program to ensure TSSM are detected before the webbing appears.

Foliar aphids now present a significant challenge. Biopesticide options include horticultural oils such as SuffOil-X and PureSpray FX, and insecticidal soaps such as Kopa and Opal. A wide variety of sprayers can be used to apply these and microbial biopesticides. While makes and models vary, they essentially fall into two types: hydraulic sprayers and low volume sprayers, and they use different methods to produce and deliver spray droplets onto the plant. 

For all biopesticides though, achieving good spray coverage is critical to efficacy. As most pests occur on the undersides of leaves, sprays must target that area to ensure sufficient contact with the pest. Investment in quality, reliable spray equipment, correct selection of nozzles and spray pressure, sprayer maintenance and calibration, and attention to applicator training, all help to achieve better efficacy from each spray application. Incorrect timing and poor coverage are primary causes of biopesticide failure.

On the BCA front, different aphid parasitoids are recommended for targeting different aphid pests, and correct identification is necessary to select the appropriate parasitoid for release. Notable foliar aphid pests include those categorized as 'small' species (i.e. green peach aphid, cotton/melon aphid), and those considered to be 'large' species (i.e. cannabis aphid, potato aphid, foxglove aphid). Aphidius colemani or A. matricariae are the best options for 'small' aphids; A. ervi for large species. Predators are less discriminating, and Aphidoletes and Chrysoperla (lacewings) eat everything; as such, they often do an excellent job of cleaning up infestations. However, Aphidoletes will not establish under artificial lights. Orius will feed on aphids as well, and when established, will help keep several other pests in check. 

Rice root aphid infestations can become problematic at this stage unless regular, preventative measures are taken. Drenching the root ball with entomopathogenic fungi is one of the few approved ways to manage aphid forms that feed on roots. The fungal suspensions must be applied in sufficient volumes to saturate the root ball and growing medium to ensure contact with aphids on the roots. Root drenches are best combined with other measures targeting winged adults (i.e., biopesticide sprays) as aphids move from the soil and into the foliage. Targeting both life stages delivers a one-two punch for greater overall efficacy.   

Flowering to harvest
The final three weeks of production can be a nail-biting period. Given the sensitivity of the trichomes and the need to consider microbial 'loadings' in flowers (owing to mandatory microbial testing requirements), there are few pest and disease management options available. For example, it is strongly recommended that growers halt microbial sprays within three weeks of harvest, which means that Beauveria-based products should not be used. Potassium bicarbonate sprays should also be halted three weeks prior to harvest to avoid potential damage to the trichomes. SuffOil-X sprays (at the 1% rate) can generally be continued up to one week prior to harvest, but different cannabis varieties show different sensitivities. Pay close attention to the plants to see if there is evidence of phytotoxicity; test first on a small batch if unsure. BCAs can still be used, but if the earlier measures have been taken and maintained, pest numbers should remain low through the final three weeks.

One of the biggest challenges at flowering is Botrytis. With the recent Health Canada decision to allow sprayable nutrients to be applied in cannabis, use of such materials can help promote plant health. Use of sprayable nutrients like ON-Gard or ON-Gard Calcium can enhance uptake of elements like calcium which is an essential component of plant cell walls. The bottom line is that stronger cells, especially in flowers, can reduce susceptibility to diseases like Botrytis. This has been demonstrated in petunia whose flowers are very susceptible to grey mold; susceptibility in this instance is linked to poor movement of calcium into flowers resulting in deficiencies, which translates to weaker cell walls, and increased susceptibility to Botrytis. Although this link has not been definitively established in hemp or cannabis, it seems to hold a lot of promise for the future as another tool to prevent infection. In the interim, there are a few sprayable materials that can help suppress the disease. These include Regalia MAXX biofungicide, which can be applied up to harvest (as a plant extract, it contains no living organisms); RootShield HC and PreStop, which can be applied as foliar sprays up until three weeks before harvest; and MilStop, which may be applied through the early flowering stages. All may be rotated within the same program to bring different modes of action to bear. 

Foliar sprays of hydrogen peroxide (e.g. ZeroTol) may be one of the most valuable tools at this time. Peroxide can be integrated into a preventative phytosanitation strategy through the vegetative growth phase and may be continued for the last two weeks through flowering to harvest. The peroxide helps reduce microbial contamination on the flowers and helps to suppress pathogens.

Some take-homes
No two locations are the same, so it is impossible to provide a single plant management blueprint that can be applied everywhere. The information shared in this two-part article series has considered many of the common challenges and some possible solutions that will help you through the production cycle. 

Remember, there are many lessons that have been learned in other horticultural crops (particularly floriculture) and techniques that may be transferred easily into cannabis production. Commitment to continued education and discourse will help the industry as a whole to move forward. 

We have discussed many preventative strategies that have been shown to work. Putting together a successful program requires planning (before the crop, not during!) and good record-keeping. There is always a lot to learn from previous crops: when pests/diseases appeared, what worked, what did not (and why), and what happened in different varieties. Good planning is fundamental to success.  

Some key points to remember 

• There are a limited number of control options available
• We do not have any 'rescue' products
• Effective strategies rely on sanitation and use of a range of preventative measures
• Look after mother, use clean cuttings, and keep a clean growing environment
• Apply preventatives early and heavy up-front
• Start clean and stay clean

By Michael Brownbridge, PhD, is a biological program manager at BioWorks Inc. He can be reached at mbrownbridge@bioworksinc.com.

Source: Greenhouse Canada

Photo by CRYSTALWEED cannabis on Unsplash

Source: Greenhouse Canada