Integrating Control of Botrytis and Powdery Mildew in a Greenhouse Crop Progress Report — June 1997
Date:5/28/97
Title of Project: Integrating Control of Botrytis and Powdery
Mildew in a Greenhouse Crop
Institution where work is being conducted: Michigan State University,
Cornell University, Texas A&M
Amount of Endowment Grant: $50,000
Covering Period: 7/96 to 7/97
Anticipated Date of Project Completion /Final Report: 7/99
Individual(s) Conducting Project:
(List Project Leader First)
M.K. Hausbeck- Title Assistant Professor
Telephone Number: 517-355-4534
M. Daughtrey
L. Barnes
and Downy Mildew in Flower Crops
Univ.), M. Daughtrey (Cornell Univ.), and L. Barnes (Texas A&M)
Investigate the epidemiology of powdery mildew. According to scanning
electron microscope observations, germ tubes (early hyphal growth of a
germinating conidium or spore) of powdery mildew on poinsettia foliage
were initiated within 2 hours of inoculation. However, germination as defined
in this study (a conidiurn displaying a primary germ tube that was equal
in length to at least half the width of the conidium) for light microscopic
observations was not observed until 6 hours after inoculation (31.7%).
Conidial germination peaked (76.0%) 36 hours after inoculation. Conidial
germination increased linearly with time following inoculation. The r2
for regression of conidial germination against time was 0.71 (P < 0.001)
and was described by the equation y = 21.7 + 1.48(x). Conidia appeared
to have a rough texture except at their rounded ends.
Appressoria (the structures that powdery mildew uses to penetrate the
plant) appeared slightly lobed to lobed and were observed on 90.0% of Terminated
conidia 6 hours after inoculation. At the last observation (48 hours),
99.0% of the germinated conidia had formed an appressorium.
The emergence of a germ tube from a primary appressorium was common
while branching of germ tubes was rare. At 24 hours after inoculation,
40.0% of germinated conidia displayed one or more secondary germ tubes.
At the last observation (48 hours), 71.7% of germinated conidia with an
appressorium had formed one or more secondary germ tubes. The percentage
of germinated conidia with an appressorium and one or more secondary germ
tubes increased linearly with time. The r2 for regression of the percentage
of germinate onidia with an appressorium and one or more secondary germ
tubes against time was 0.92 (P < 0.001), and was described by the equation
y = -9.2 + 1.78(x).
Haustoria (the structures that powdery mildew uses to absorb nutrients
from the plant) appeared as globose structures with the haustorial sac
typically visible. Within 24 hours after inoculation, 32.5% of germinated
conidia had an appressorium and haustorium. Germinated conidia with an
appressorium and haustorium increased to 5 1.0% at 48 hours. The percentage
of germinated conidia with an appressorium and haustorium increased linearly
with time, and the r2 for regression of the percentage was 0.93 (P<
0.001), and was described by the equation y = -6.0 + 1.26(x).
In colonies two weeks old and older, germ tubes formed from up to six
positions on germinating conidia. Hyphae maintained directional growth
from the point of their initiation, following the contours of the host
surface. Branching hyphae grew in a direction obtuse to the line of direction
of hyphal growth. Lateral appressoria that formed off of hyphae appeared
singly and in pairs opposite one another, appearing slightly lobed to lobed.
Cuticle degradation was visible beneath secondary appressoria. Conidiophores
(specialized hyphae of powdery mildew which produce conidia) displayed
an arced basal cell and consisted of up to four cells, including the developing
——- conidium. Thin, slightly spiraling parallel longitudinal ridges
were occasionally visible on the cell directly adjacent to the conidium.
Conidial germination was significantly reduced when the incubation temperature
was 30 C 64%) versus 20′C (> 80%) although incidence of appressoriurn formation
from germinated conidia was not significantly affected. Significantly more
conidia were shriveled when incubated at 30′C versus 20′C for 96 hours.
Incubating conidia at 30′C significantly limited the incidence of secondary
germ tube formation on germinated conidia (< 1%) compared with incubating
conidia at 20′C (> 53%). Similarly, haustorium development was significantly
limited by the 30′C incubation treatment (< 4%) compared to the 20′C
incubation treatment (> 32%).
Temperature manipulation may be a useful tool in managing powdery mildew
on poinsettia. Germination was significantly reduced when conidia were
exposed to 30′C. Although formation of appressoria from germinated conidia
was not affected by the 30′C heat treatment, development of germ tubes
and haustoria was hatted and shriveling of conidia was observed.
High temperature eradicative treatments might be feasible when vegetative
growth desired such as during stock plant production or following rooting
of cuttings. It may be advantageous for rooted cuttings to be exposed to
a high temperature eradicative treatment since the plant size is small,
requiring treatment of a much smaller area than if treating larger stock
plants. While these treatments would not be helpful in a situation where
reinoculation is possible, it may be helpful in situations where propagators
are interested in treating rooted cuttings prior to shipment to insure
the cuttings are free of powdery mildew. Though it is unlikely that heat
treatments can be used alone, there may be the potential to incorporate
temperature manipulation with scouting and fungicide applications to further
reduce growers’ powdery mildew management costs.
Future plans. Expand research on the temperature levels that
inhibit powdery mildew on poinsettia and gerbera. Initiate fungicide trials
on powdery mildew on gerbera. Investigate longevity of fungicides applied
to stock plants in protecting subsequent cuttings against powdery mildew.
