Rainfastness of Fungicides in Strawberry: What factors should be considered when using Contact or Systemic Fungicides.
by Aaron Cato - March 18, 2020
Do you have rain in your future and strawberries that need to be protected from
disease? Here’s some info about rainfastness and when to use contact vs. systemic
fungicides.
With the large amount of rain we’ve already seen and what’s forecasted for the following
week, we’ve received a lot of questions about rainfastness of fungicides and what
factors to consider when choosing a contact or systemic fungicide.
If you have further questions about how a fungicide moves within a plant, or about
specific terminology such as translaminar vs. acropetal, please check out this Fungicide 101 ”resource from Iowa State.
Contact vs. Systemic Fungicides
When we talk about fungicides we often classify them by mode of action (MOA) or FRAC
group, and then more specifically by different chemical and trade names within that
FRAC group such as azoxystrobin or trifloxystrobin. Another set of terminology we
often use is contact (protectant) vs. systemic fungicides. Breaking fungicides down
into contact vs. systemic is helpful because it often lets you know a little about
their activity, whether they have any ability to be curative rather than only preventative,
and it helps us understand whether these fungicides are moving in to plant tissues.
Contact (Protectant) Fungicides
Contact fungicides are those that do not move into plant tissues but instead spread
out on the surface where spray droplets are deposited. Any area on leaves or flowers
that are not covered by these contact fungicides will not be protected from potential
infection. It’s also important to note that any foliage that is produced after an
application is made will not have any protection when a contact fungicide is used
alone.
Contact fungicides work best when applied during sunny and dry conditions, where they
can quickly dry on leaves and create a barrier to moving spores and disrupt the germination
of present fungus. When allowed ample time to dry (at least a few hours), these fungicides
will continue to add protection until rain events occur, with some level of wash-off
expected with even a light mist. In general, expect a 1 inch rain event to wash away
around 50% of residue and a 1-2 inch rain event to wash most of it away. Even with
only losing 50% of residue after a 1 inch rain event, this could mean a significant
portion of strawberry plants are potentially susceptible to infection.
Strawberry plant with some Captan residue still present after light rainfall.
Systemic Fungicides
Systemic fungicides are those that not only spread out on surfaces that they contact
but also move within plant tissue. This means that not only do these products prevent
spore germination and infection where they contact, they have potential to kill mycelium
and fruiting bodies in leaves and other structures within the plant where they are
able to travel. Systemic fungicides are often classified by the manner and degree
in which they move within plants. Translaminar movement refers to fungicides that
are only locally systemic whereas acropetal (xylem or phloem) movement refers to fungicides
that can potentially move further throughout the plant.
These fungicides work best when applied during cloudy and humid conditions where the
leaf cuticle is generally swollen and more permeable, which allows these systemic
fungicides to more readily move into tissue. During dry and sunny conditions, the
leaf cuticle is generally flatter and less permeable, which not only limits the fungicides
ability to enter leaf tissue, but also leads to increased breakdown by sunlight, heat,
or microbial activity.
Systemic fungicides can generally be expected to be more rainfast because washing
the material off just the surface does not fully remove them from the plant. However,
we still need to apply these fungicides with ample time before a rain event. We need
to plan to apply these fungicides at least 12 hours before rain events, and a rain
event within 2-3 hours could be expected to have removed most of that fungicide.
When to Consider Contact vs. Systemic Fungicides in Strawberry
When considering a protectant fungicide application for your strawberry we need to
consider many factors. First, consider the label instructions and restrictions. Next,
consider whether this product is effective for controlling Botrytis fruit rot or anthracnose
fruit rot. These are the two main diseases you will deal with, and any fungicide application
that is being applied needs to include at least one product that has efficacy in preventing
these diseases. This is extremely important when considering any resistance that could
be present within the fungus on your strawberry that could make these fungicides ineffective.
Please reference the Southeast Regional Strawberry IPM Guide for information regarding product efficacy.
There are a few things to consider when choosing a contact or systemic fungicide before
rain events, and when considering their residual effectiveness after rain events have
occurred.
If there is a significant rain event forecasted in the next 24 hours, don’t rely on
contact fungicides such as captan or thiram alone, and instead consider a systemic
fungicide. Systemic fungicides will provide better residual effectiveness after rain
events, as long as enough time was allowed for those fungicides to penetrate plant
material.
If you used a contact fungicide such as captan or thiram alone and a significant rain
event occurred after, you need to apply another fungicide application to help limit
disease. If you applied captan or thiram + a systemic, you are now only getting protection
from the systemic fungicide, which may be important when considering efficacy of some
products for Botrytis vs. anthracnose.
When considering any foliar fungicide, the time from application to the next rain
event is critical. If contact or systemic fungicides were applied and a significant
rain event occurs within 2 hours, it is very likely that a large portion of that fungicide
was washed off, and no efficacy should be expected after the rain event. Both contact
and systemic fungicides may also be susceptible to some level of wash-off within 12
hours of application.
See below for a list of fungicides that could potentially be used for prevention of
Botrytis and anthracnose fruit rot (efficacies range from Excellent – Fair). Remember
to consider legality, efficacy, economics, and timing first when choosing these fungicides.
Then think about whether you should rely on a contact protectant or need a systemic
fungicide to potentially weather the coming storms. In general, plan to spray effective
fungicides at least every 7 days, and increase spray intervals if rain events are
excessive.
Contact and Systemic Fungicides in Strawberry Commonly used for Botrytis and Anthracnose
Fruit Rot Control
Fungicide
MOA (FRAC Group)
Contact or Systemic*
Captan; Captec
M04
Contact
Captevate
M04 + 17
Contact
Thiram
M03
Contact
Fracture
BM01
Contact
Topsin M
1
Systemic
Rovral
2
Systemic
Tilt; generics
3
Systemic
Fontelis
7
Systemic
Kenja
7
Systemic
Scala
9
Systemic
Pristine
7 + 11
Systemic
Merivon
7 + 11
Systemic
Luna Sensation
7 + 11
Systemic
Cabrio
11
Systemic
Abound
11
Systemic
Intuity
11
Systemic
Switch
12 + 9
Systemic
Elevate
17
Contact
Ph-D, OSO
19
Systemic
*Systemic denotes that some level of movement occurs into plant tissue, but the degree
of movement varies.
** See the Southeast IPM Guide for efficacy ratings of these fungicides.
