In last week’s post, we looked at several common pests that can ruin your day as a hemp farmer if they get too close. In this article, the second part in the series, we’ll meet three of the more than 100 fungal pathogens, numerous bacteria, viruses, and parasites that could infect Cannabis.
Among this large group of insurgents, we’ve selected just three that sooner or later you are most likely to encounter.
Right now, as an organic grower you might be wondering, how can your high-CBD crop triumph over such a formidable list of potential disease-carrying pathogens?
Fortunately, very few hemp pests and diseases cause economic damage.
Still, your best defense is a good offense; like prevention and early detection through daily crop monitoring.
Should your daily scouting turn up a pest or disease problem, here’s our advice in three simple steps:
- Got CBD? Keep calm and carry on.
- Grab your copy of the most comprehensive manual on the planet, Hemp Diseases and Pests Management and Biological Control, written by one of Vermont’s own, J.M. McPartland Ph.D. and his colleagues R.C. Clarke, and D.P. Watson.
- Thumb (or scroll) to the back of McPartland, et al.’s classic to “Appendix 2; Dichotomous Key of Diseases and Pests”. Their easy to follow key takes you step-by-step through a symptom identification procedure that covers over 90 common Cannabis problems found in the field.
Read on, as we offer descriptions and recommendations on how to avoid the three most common hemp diseases or bring them under control.
Top 3 Common Hemp (Cannabis) Diseases
This is the bane of hemp growers; the dreaded “bud rot”, easily identified and nearly impossible to combat. The culprit is Botrytis cinerea, a fungus that affects hundreds of plant species, including Cannabis. It is especially common in regions like the Northeast where humidity, a rainy harvest period, and cool to moderate temperatures prevail.
Gray mold can attack hemp seedlings, which leads to damping off. If you catch it early, close observation might reveal a change in the color and texture of the plant. For instance, leaves may develop brown or black spots before drying out completely, and the stems turn brown and ulcerated.
In most cases, however, the tell-tale cottony gray or brown fuzz of Botrytis first becomes evident in the compact, mature buds of late summer. (see gallery below) For late-flowering cultivars or those that produce large, dense buds, Botrytis attacks can be devastating. Yes, we know large, dense buds are desirable, but…
Once identified, immediate action is needed to prevent the disease from spreading – an attack of Botrytis can wipe out a crop in a week.
According to McPartland, “Breeding resistant plants is the ultimate solution for grey mold. (Mexican, Colombian, and Thai) plants rarely suffer from bud rot, whereas the dense, tightly packed buds of Cannabis afghanica tend to hold moisture and rot easily.”
Another easy to identify and notorious hemp pathogen is hemp canker or white mold. The culprit is the fungus Sclerotinia sclerotiorum. White mold thrives in cool, moist conditions, just like gray mold. Hemp canker can affect its host at any stage of growth, although symptoms usually begin in late summer on full-grown plants. (see gallery below)
About hemp canker, McPartland writes, “Symptoms begin as lesions on stems and branches of plants nearing maturity. The lesions collapse into cankers and become darkly discolored. Affected areas take on a shredded appearance, and the pith becomes filled with white cottony mycelium. Plants remain in this condition or wilt and fall over.”
Doesn’t sound good, does it? S. sclerotiorum doesn’t give up easily and can even spread through the harvested products.
Once Sclerotinia infections become visible, the internal damage to the plant is often too severe to treat. At this point, it’s best to remove the infected plants or branches to prevent further crop contamination.
Recommendations For Controlling Gray Mold And White Mold In Hemp:
- Practice crop rotation. This is the best way to avoid disease build-up in the soil. Alternate year-to-year between broadleaf plants (hemp is a broadleaf) and grasses or small grains over a four-year rotation.
- Deep autumn plowing (3+ inches) and spring tillage done correctly expose pests and pathogens to their natural enemies and the weather.
- Mold prevention starts with good airflow, so be sure your field layout is leaving sufficient space between the plants. Prune off the lowest branches.
- Keeping the area around the plants and in the aisles clear of weeds has many benefits and reduces opportunities for pests and disease to get a foothold. Weeds also hold humidity and restrict airflow. Cover crops kept mowed can help a lot.
- Research has shown that disease-carrying spores are easily splashed or blown from weeds to crops. Avoid using a weedwacker for this reason.
- Choose cultivars with a short flowering cycle to harvest early and avoid persistent autumn rains.
- If the disease is isolated on the plant, prune infected branches or flowers and remove them far from the field to prevent further contamination.
- Off-site burning or burial of the material may be the best option.
The genus Fusarium is said to contain 1,000 species of soil-dwelling fungi, giving it multiple ways it can infect Cannabis; from Fusarium stem canker, and root rot, to Fusarium wilt, and damping off of seedlings.
In each case, Fusarium causes havoc in its host by blocking the flow of water and nutrients throughout the plant, leading to systematic death, section by section. (see the gallery below)
If Fusarium stem canker strikes, it usually gets a hold on mid- to late-season hemp. Symptoms begin as lesions on the stem. As the disease progresses, cankers enlarge, turn dark brown to black, and may split open. Leaves above the canker wilt and die.
In the case of Fusarium wilt, dark spots appear on lower leaves first, turning from yellow to brown before the leaves collapse. Leaf and stem then droop downwards but remain attached, and this pattern repeats, moving up the plant.
With Fusarium root rot, on the other hand, its symptoms may not be seen until it is too late. Cannabis roots can turn red, with the stem turning brown at the soil line, and this coloration travels up the stem. Lesions open in the stem, inviting insects and disease, eventually killing off the plant.
Recommendations for Controlling Fusarium in Hemp:
In addition to the suggestions above for controlling gray mold and white mold, try the following:
- Practice a high level of hygiene in the greenhouse. Use an agricultural sanitizer to disinfect reused pots and growing trays.
- Avoid overwatering (always recommended for Cannabis) and allow the soil to dry out between waterings.
- Avoid excess nitrogen, which increases the incidence of bacteria, fungi, and leaf-eating pests.
- Avoid planting in poorly drained or heavy clay soils. If that’s unavoidable, set your plants into raised rows.
- Research indicates that inoculating the soil with beneficial bacteria or Trichoderma and mycorrhizal fungus, not only improves plant nutrition; these inoculants protect plants from a variety of root pathogens.
For a full account of useful countermeasures, see McPartland et al., “Chapter 9: Cultural & Mechanical Methods of Controlling Diseases and Pests”.
Did I say we had just three prominent pathogens to feature? Here are three more which can sometimes be catastrophic, and whose symptoms often mimic those above:
Always keep in mind, in addition to a daily routine of scouting your crop your best defense against pests and disease is:
- Keep a sanitary greenhouse,
- Build healthy soil with lots of organic matter,
- Use beneficial inoculants, and
- Grow vigorous plants.
Then, as the crop matures, hemp’s natural defenses and higher numbers of natural pest predators make an economically damaging event less likely.