2023 Guide to Hemp Production - 14

PLANT HEALTH
PREVENTING
HEMP DISEASE
Recognizing and understanding gray mold and powdery mildew of hemp can help
growers manage these diseases.
BY C. M. OCAMB
T
he fungus, Botrytis cinerea, is a common
pathogen of plants and infects many different
crops worldwide. It causes gray mold, also
known as Botrytis bud blight and stem canker, on
hemp (Cannabis sativa L.). This fungus initially infects
weak, damaged, or senescing leaf, bud, or stem
tissues, especially when environmental conditions are
favorable. Temperatures between 50 and 68° F and a
high relative humidity (> 95%) or free water on plant
surfaces favor infection of plants and subsequent
sporulation by Botrytis. Once infection has occurred,
the fungus can continue colonizing plants when the
temperature is 32 to 80° F.
Hemp grown inside greenhouses is at higher risk
for gray mold throughout plant development due to
the higher relative humidity that occurs inside. Rainy
periods, especially during flowering and bud maturation,
can result in significant disease in outdoor plantings or
inside greenhouses with poor ventilation and heating.
Botrytis produces a gray-brown mass of asexual
spores known as conidia on infected tissues; the
spores are dispersed by wind currents as the relative
humidity drops. Conidia can be produced throughout
the growing season, depending on environmental
conditions. Small, black hyphal structures known
as sclerotia can also develop on or inside affected
plant tissues, although they occur infrequently with
Botrytis. These durable sclerotia are released from
decomposing plant tissues, fall onto or are mixed into
the soil, where they survive for months to years. There
may be a widespread presence of B. cinerea in the
local landscape (ornamental plants, herbs, fruit crops,
vegetables, etc.) throughout the year, depending on the
local climatic conditions, so sclerotia are not necessary
for Botrytis outbreaks in hemp.
The economic impact of Botrytis on hemp grown
for cannabinols, buds or seed is primarily through
the infection of flowers and bud tissues. Tissues will
14 | 2023 GUIDE TO HEMP PRODUCTION
initially exhibit a water-soaked appearance where
infection has occurred. Infected buds eventually turn
brown as the affected tissues die, and the fungus
often produces spores on dead tissues (Fig. 1A-C).
Botrytis can also infect stems with pale to light-brown
colored areas developing as well as gray-colored
sporulation (Fig. 1D) under conducive conditions.
Stem cankers can result in splitting or breaking of
branches as well as result in death of the growing tip
on affected branches. Infected hemp tissues can be
killed rapidly, potentially within one week. Young hemp
seedlings can also be killed by Botrytis.
A
B
C
D
Figure 1. Hemp bud in early stage of gray mold
development on a cola, denoted by the brown-colored
tissues (A), a hemp plant with bud blight due to gray mold
with multiple affected colas (B), a hemp bud infected with
Botrytis that is exhibiting gray-brown sporulation (C),
and a portion of a hemp stem with a Botrytis canker and
profuse gray-colored sporulation (D) characteristic of this
disease. Photos: C.M. Ocamb, Oregon State University.
C
Management of Gray Mold in Hemp
* Minimize conductive conditions for disease by
increasing plant and row spacing to promote air
movement within and around plants. In greenhouses,
2023 Guide to Hemp Production

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