Using Organic Fungicides
Many homeowners and even nursery professionals, when confronted with
a disease problem, stress that they wish to apply an “organic” fungicide
to control the disease, because it is more environmentally friendly.
Others are equally adamant that they want something that works, regardless
of the ecological impacts. Many of the organic fungicides fulfill these
two most important criteria in disease management in one bottle: Efficacy
with minimal ecological impact. However, it must be stressed that improper
use of any chemical, synthetic or organic, will result in a failure
of control and have negative environmental consequences.
Effective use of organic fungicides (or any fungicide for that matter)
requires a solid strategy of integrated plant management. As opposed
to focusing on the pest or pathogen in a traditional integrated pest
management (IPM) program, integrated plant management focuses on identifying
the right plant for the site and management strategies aimed at keeping
that plant healthy. The backbone of integrated plant management includes
carefully matching the plant to the soil type, sunlight levels and
watering conditions; proper sanitation; appropriate fertilization and
pruning,
when necessary. All of these strategies work together to prevent disease
problems from developing.
Prevention is the key word here and it is essential to successful
organic gardening because established populations of plant pathogens
don’t
respond well to any chemical, organic or synthetic. The fungicides and
bactericides discussed herein all have low acute toxicity to both birds
and mammals (including humans). This doesn’t mean you should
use them indiscriminately. Copper and sulfur-based chemicals should
be used
with caution for the sake of your plants and in the case of the copper
based fungicides, to prevent groundwater contamination (copper is a
heavy metal and toxic to fish). As always, follow label instructions
carefully.
Prior to applying any chemical, make certain of your diagnosis. If
you are certain of your diagnosis, proceed to the next step of identifying
which pesticide is best for the problem you wish to manage. Remember
to determine if the plant you wish to treat is labeled and that no
contradictions
exist. Finally, recognize temperature limitations and apply during
appropriate weather conditions. When applying any chemical, be sure
to protect yourself
using the proper precautions.
Sulfur
The oldest recorded fungicide is sulfur. It has been used for
over 2000 years. Early in agricultural history, Greeks recognized
its efficacy
in controlling rust on wheat. Although few homeowners grow their
own wheat, sulfur can be used as a preventive fungicide against powdery
mildew,
rose black spot, rusts and other diseases. The key to its efficacy
is that it prevents spore germination. For this reason, it must be
applied
prior to disease development for effective results. Sulfur can be
applied
as a dust or purchased in liquid form. It is also available as a
wettable powder, which allows you to decide how you wish to apply it.
Do not
use sulfur if you have applied an oil spray within the last month – the
combination is phytotoxic (plant-killing). Likewise, remember to not
use sulfur when temperatures are expected to exceed 80°F. Finally,
there are certain “sulfur-shy” plants including varieties
of gooseberries, currants, apricots, raspberries and cucurbits that
should never be treated with sulfur.
Lime-sulfur is formed when lime is added to sulfur to help it penetrate
plant tissue. It is more effective than elemental sulfur at lower concentrations:
however, the odor of rotten eggs usually discourages its use over extensive
plantings.
Copper
Although not the oldest fungicide, Bordeaux mixture has been
successfully used for over 150 years, on fruits, vegetables and ornamentals.
Unlike
sulfur, Bordeaux mixture is both fungicidal and bactericidal. As
such, it can be effectively used against diseases such as leaf spots
caused
by bacteria or fungi, powdery mildew, downy mildew and various anthracnose
pathogens. The ability of Bordeaux mixture to persist through spring
rains and to adhere to plants is one reason it has been so effective.
Bordeaux mixture contains copper sulfate, which is acidic, neutralized
by lime (calcium hydroxide), which is alkaline.
Bordeaux mixture comes in several formulations. One of the most popular,
effective and least phytotoxic formulations for general home garden
and orchard use is the 4-4-50 formulation (These numbers translate into
four pounds copper sulfate and four pounds lime in 50 gallons of water).
For determining a more useful amount, consult the Ohio State Bulletin:
Controlling Insect and Disease in Home Fruit Plantings.
These formulations were developed in recognition of the fact that
copper, like sulfur, is phytotoxic and that the level of toxicity is
related
to the age of plant tissue being treated. If Bordeaux mixture is applied
in spring after the tree breaks dormancy (to prevent infection by the
fire blight bacteria, Erwinia amylovera), a weaker, more dilute formulation
should be used to reduce the risk of plant injury.
Some labels will
recommend twice as much fungicide, which translates into an 8-8-50
formulation,
for other diseases or late season application. Application of Bordeaux
during hot weather (above 85°F) may cause yellowing and leaf drop.
Additionally, leaf burn can occur if it rains soon after a Bordeaux application.
Spray oil can be added to the spray mixture to reduce the likelihood
of burn. Bordeaux mixture can be applied as either a dust or purchased
as a liquid formulation. Some sensitive plants require diluting the product
to one half strength (depending on the product used – see label)
to avoid phytotoxicity. These include geraniums, ivy, pansy, celery,
strawberry, azaleas, dogwood and juniper.
Care should be taken when
applying this fungicide to young, tender leaves of apple, pear, plum
or rose.
Do not apply Bordeaux to corn or ornamental sorghum, which are described
as copper-sensitive plants.
Source: Janna Beckerman, Ph.D., University of Minnesota
Extension Service Yard and Garden Line newsletter
Spring 2003
Hort
Shorts | Hort Tips | Crabgrass:
A Weed’s Weed! | Health & Household
Tips | Perennial Plant of the Year 2003 | Mulch
Reminder | Using Organic Fungicides | Bug
Bites: A Bug Bite is Not Always a Bug Bite | Lawn
Care Calendar | Cybergarden Sites | Pondering
Early Planting? Think Lettuce | Eggology | Soup
for Supper | Protein-Rich Diets and Weight
Loss
Index
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