Linda Chalker-Scott, Ph.D., Extension Urban Horticulturist and Associate Professor,
Puyallup Research and Extension Center, Washington State University
The Myth of Milk and Roses:
"Milk sprayed onto rose leaves will prevent fungal and bacterial diseases"
The Myth
In the last few years, the Internet has been abuzz with the news that spraying milk on rose leaves can
control foliar diseases. The web stories most often cite a Brazilian study published in 1999 focusing on
powdery mildew control on zucchini. This new alternative to conventional fungicides has been
augmented with anecdotal reports of successful powdery mildew control on a variety of plants, including
roses. Moreover, the treatment is also touted as preventing leaf black spot, thus giving hope to rose
aficionados everywhere of a safe, effective method of growing disease-free specimens.
The Reality
Milk has been part of the horticultural toolbox for many decades; for instance, it has been used with
varying effectiveness as a spreader or sticker in pesticide applications. Perhaps the best-documented use
of milk has been in reducing the transmission of leaf viruses, especially tobacco mosaic and other mosaic
viruses. Studies over the last half of the 20th century document the effectiveness of milk used for this
purpose (Table 1):
Table 1: Effectiveness of milk products in protecting leaves from viruses:
Location Year Crop Milk product Conditions Effective?
Australia 1967 Sugarcane Evaporated Field Somewhat
Brazil 2001 Zucchini Raw + leaf extract Field Yes
Brit. Col. 1964 Tomato Unknown Unknown Yes
Florida 2004 Hibiscus Non-fat dry Field Yes
Germany 1971 Spinach Full cream Greenhouse Yes
Beans Full cream Greenhouse Yes
Beets Full cream Greenhouse Yes
Celery Full cream Greenhouse Yes
Pea Full cream Greenhouse Yes
Potato Full cream Greenhouse No
Hawaii 1994 Orchid Nonfat Laboratory No
India 2003 Sunflower Nonfat Field Somewhat
Manitoba 1959 Barley Nonfat Field Yes
New Zealand 1943 Tobacco Unknown Field Yes
Quebec 1968 Tomato Unknown Unknown Yes
Russia 1961 Tobacco Whey Laboratory Somewhat
Taiwan 1991 Pepper Nonfat Field No
The effectiveness of milk sprays in reducing virus transmission probably improves with the concentration
of the milk product used; milk concentrations of 20% nonfat dry or 30% fresh were reported as effective
in the Florida and Brazil studies, respectively. Milk is routinely recommended as an organic hand
sanitizer when handling virus-susceptible seedlings for transplant.
How milk functions as an antiviral agent is not clear but there are a few attractive hypotheses. First, milk
may deactivate viruses chemically or isolate them physically; hence the success of milk as a sterilizing
treatment. Second, milk may prevent aphid attack, and thus transmission, of aphid-borne viruses.
Aphids may be deterred by the milk film on the leaf or attacked by aphid pathogens whose growth is
enhanced by milk sprays; a 2003 study identified just such a fungal agent on treated pepper leaves.
Recently, milk has made an appearance as an antifungal agent, specifically in powdery mildew prevention
(Table 2). [Note that there have been no published scientific studies investigating roses or any other
ornamental plant species.]
Table 2: Effectiveness of milk products in protecting leaves from powdery mildew:
Location Year Crop Milk Conditions Effective?
Australia 2000 Melon Full & half Field Somewhat
Brazil 2005 Pumpkin Raw Field Yes
Pumpkin Pasteurized Field Yes
Brazil 1999 Zucchini Fresh Greenhouse Yes
Italy 2003 Cucurbits Unknown Field Inconclusive
Italy 2002 Cucurbits Fresh Greenhouse Yes
Cucurbits Dried Greenhouse Yes
Cucurbits Fresh Field
Yes/No
Cucurbits Dried Field Yes/No
UK 2003 Wheat Pasteurized Greenhouse Yes
The results of these studies suggest that milk treatment under controlled (greenhouse) conditions is more
successful than in the field. None of these studies utilized nonfat milk, so it’s unclear whether it would
show any efficacy. In general, it appears that milk applied before fungal inoculation is more effective
than milk applied after infection is present. Stems and lower leaf surfaces may be less protected,
especially under high disease incidence.
This last point is important when considering the value of anecdotal claims of the effectiveness of milk or
any other pesticidal treatment. Unless plant material is actually challenged (exposed to) the disease or
pest of interest, it is impossible to attribute the subsequent lack of disease or pests to that treatment.
Statements such as “Last year I had horrible black spot problems, but this year I used milk spray and my
roses are disease-free” display faulty logic in the assumption of cause and effect where none may actually
exist.
There are a few potential drawbacks to using milk as a foliar spray:
•
Milk-fat can produce unpleasant odors as it breaks down.
•
The benign fungal organisms that colonize leaves and break down milk can be aesthetically
unattractive.
•
Dried skim milk has been reported to induce black rot, soft rot, and Alternaria leaf spot on treated
cruciferous crops.
Is it worth trying milk as a treatment for viruses, powdery mildew, or any other disease? Absolutely!
There is substantial evidence that milk treatments can be effective in the protection of some crops, and
organic farmers especially might benefit from this method. But on which plant species will milk
treatment prevent disease? What pathogens are actually inhibited by milk products, and which milk
products are the most effective? Until these questions have been answered, it will be impossible to devise
a reliable application protocol.
The Bottom Line
•
There is no evidence that milk sprays are effective in controlling black spot on roses or any other
ornamental plant species.
•
Milk sprayed onto leaves may act as a nutrient source for benign microorganisms, decreasing the
leaf area available for powdery mildew to infect.
•
Leaves coated with a milk spray may be less vulnerable to aphid attack, thereby reducing the
transmission of aphid-borne viruses.
•
Milk sprays can encourage the growth of other microorganisms, whose presence may be
aesthetically unappealing.
•
Milk sprays may be a viable alternative to conventional pesticides, especially for organic farmers.
For more information, please visit Dr. Chalker-Scott’s web page at
http://www.theinformedgardener.com.
