Milk as a Management Tool for Virus Diseases

There are currently no available chemicals for controlling virus diseases in infected plants. However, milk as a spray or dip for seedlings, has often been suggested as a means of reducing the incidence of virus infections. This idea originated from several older studies (1940s and 1950s) that have demonstrated milk's effectiveness in reducing infection due to tobacco mosaic virus (TMV) in pepper, tomato, and tobacco. Research in field tomatoes in Mississippi in the late '50s suggested that milk, as a dip for fingers prior to handling diseased seedlings, reduced TMV incidence from over 50% to 0% in pepper, and from 90% to 15% in tomato. Although the majority of these studies have focused on TMV, infection due to other viruses can also be reduced, but to differing degrees. Such viruses include pepper mild mottle mosaic virus (PMMV), cucumber mosaic virus (CMV), bean mosaic virus (BMV), and tobacco ring spot virus (TRSV).

Mode of Action

Early work led to several hypotheses about milk's mode of action. One worker in the '30s suggested that milk inhibited infection by somehow lowering the plant's susceptibility to the virus. Other work in the '40's suggested that milk "inactivated" the virus by forming a loose "molecular union" which, if broken, results in re-activation of the virus. That is, the inhibiting effects were reversible and the effect was on the virus and not the plant. Then work by an Australian scientist in the '50s supported the earlier hypothesis that milk contains a substance that inhibits infection by its effect on the plant, i.e. by supposedly inducing some type of resistance. He also found that the inhibitory effects were restricted to those parts of the plant to which milk was applied. Furthermore, his lab investigations suggested that the active substance in the milk was a protein. The conclusion that the active substance is a protein component or number of such components is supported by recent work carried out by USDA scientists. But the answer to how exactly milk inhibits or reduces infection by viruses is still not known.

Effective Formulations

The various milk products are not equally effective. Several workers report that skim milk and whey are equally effective as whole milk. However, one worker suggests that evaporated milk and buttermilk are not as effective as these other milk products. There is also the finding that pasteurization, sterilization, or boiling did not appreciably destroy the inhibiting properties of effective milk products. Skim milk formulations should contain at least 3.5% protein, and may be used either fresh or reconstituted in water from skim milk powder. The only caveat to the application of milk to plants is the potential development of sooty mould on treated foliage.

Practical Implications

Generally, hands and cutting implements should be dipped in milk between plants to reduce spread of virus. Note, however, that use of milk will not cure an infection, nor provide lasting protection from contact spread, nor prevent spread by insects. It will only reduce spread via contact. Chemical disinfectants known to kill viruses at specific concentrations are likely a more reliable form of disinfection, particularly for tools, to reduce the spread of virus diseases. Yet, there is the appeal of milk, a natural product not known to be phytotoxic, and that offers a worker-friendly solution to reducing incidence of virus infections.