2023 Guide to Hemp Production - 15

PLANT HEALTH
maintain warm temperatures and high light intensities
while keeping the relative humidity below 50%.
* Avoid practices that keep plants wet for 12 hours
or longer. Limit irrigation during bloom. Use bottomwatering
methods rather than overhead irrigation.
Prevent standing water or pooled water inside
greenhouses.
* Avoid over-fertilization with nitrogen as this creates a
more dense plant canopy that impedes air movement
and promotes higher humidity conditions.
* Avoid mechanical injuries to plants, including those
caused by insect feeding on buds, since Botrytis
more easily infects wounded tissues.
* Apply a protective fungicide program during bud
development that is efficacious on gray mold.
Contact your state department of agriculture or
Extension faculty at a land-grant university in your
state for recommended fungicides.
Powdery Mildews in Hemp
The fungi that can cause powdery mildew in hemp
include Golovinomyces ambrosiae and Podosphaera
macularis. Golovinomyces ambrosiae is the species
that most commonly infects hemp and has been
reported on cannabis in Oregon, New Jersey, and
Kentucky as well as Canada. Golovinomyces ambrosiae
may also infect members of the Asteraceae family;
it has been reported on sunflower, dahlia, daisy,
Jerusalem artichoke, zinnia and ragweed. Podosphaera
macularis more commonly occurs on hop, but it has
been reported on hemp in the Pacific Northwestern
U.S., Canada and Switzerland.
Golovinomyces and Podosphaera produce copious
amounts of asexual spores (conidia) in chains (Fig.
2). The white powdery growth that is characteristic
of powdery mildew is due to these conidia. Colonies
become visible approximately seven to 14 days after
infection. Conidia can be moved via wind currents,
endangering neighboring hemp plants or fields in the
vicinity. A high relative humidity promotes infection
while wet plant surfaces will inhibit infection by
conidia. Warm conditions are conducive for the
production of conidia and infection; around 77° F is
ideal for conidia to be produced by P. macularis on
hops, but conidia production and infection are inhibited
when temperatures exceed 86° F. It is anticipated that
temperature effects would be similar for P. macularis
on hemp. No information is currently reported regarding
conditions conducive for sporulation and infection by
G. ambrosiae in hemp.
These two powdery mildew fungi can
produce overwintering fruiting bodies known as
chasmothecia that yield sexual spores (ascospores),
but evidence of ascospores in association with
cannabis is lacking. Podosphaera macularis has
not been found to produce chasmothecia on hop
grown in the Pacific Northwest, but chasmothecia
are produced on hops in other regions. Ascospores
are forcibly ejected from chasmothecia beginning
in the spring and then move with wind currents.
Ascospores require a high relative humidity or free
moisture in order to infect susceptible host plants.
Podosphaera macularis is also known to overwinter
as mycelium on infected crown buds of hop plants in
the Pacific Northwest. These two powdery mildew
fungi may also survive/over-winter on infected host
plants in greenhouses.
Infection of hemp plants by powdery mildew fungi
can occur at any stage of plant growth, but in regions
with dry summers, onset of powdery mildew in field
production is more common during the latter part
of crop development, especially during flowering.
Infections appear as whitish, powdery spots typically
on the upper leaf surface. Entire leaf surfaces can be
covered with powdery mildew, and petioles as well as
flower bracts may be infected. As foliar infections of
Golovinomyces and Podosphaera age, if chasmothecia
are produced, they appear as tiny, roundish specks
ranging from yellow to dark-brown to black coloration
within powdery mildew colonies.
A
B
C
Figure 2. Hemp plants exhibiting powdery mildew. A plant in
the field (A) and a close-up of a leaf (B) that are infected by
Golovinomyces ambrosiae. A hemp branch with buds that is
infected with Podospaera macularis (C). Photos A and B: C.M. Ocamb.
Photo C: T.A. Bates, Oregon State University.
Management of Powdery Mildew in Hemp
* Removal of infected leaves early in disease
development can delay disease buildup but
requires routine disease scouting.
* Maintain adequate nitrogen levels but do not
over-apply because more succulent tissue is
more susceptible to infection.
* Applications of silicon (300 to 600 kg/ha) applied
to hemp roots suppressed powdery mildew in
studies conducted by the University of Kentucky.
Silicon applications may be especially helpful
for greenhouse grown plants produced normally
2023 GUIDE TO HEMP PRODUCTION | 15
2023 Guide to Hemp Production

Table of Contents for the Digital Edition of 2023 Guide to Hemp Production
