I'm fairly skeptical about almost everything. 

For years I've read stories about spraying aspirin water on plant leaves to boost immunity of plants against viruses, bacteria, and fungi.

I had two issues with that.  

First, I long believed that plants did not have an immune system.  At least, not like animals.  Plants do not have white blood cells, or any blood cells.  So no antibodies, no cells that "eat" invading diseases.  Plants do not produce the substance called "complement", which is essential in animal immunity.  Plants can't create fever, and they do not have lymph nodes, also essential in animal immunity.

Second, in animals the function of aspirin is to reduce inflammation and fever.  Aspirin also has a completely coincidental function - actually a side effect - that results in decreased function of blood cells called "platelets".  Platelets stimulate and provide structure for blood clots.  Aspirin was found coincidentally to inhibit or reduce platelet function, which results in less clot formation.  Which is why aspirin is recommended for reduction of heart attack and stroke risk, because heart attacks and strokes are usually caused by a sequence of events that includes clotting when there should not be clotting.

File:Red and white human blood cells as seen under a microscope using a blue slide stain.jpg

I returned to the question of aspirin due to effects of mosaic virus on fig trees.  I am not a horticulture scientist, so correction or elucidation is welcome.  But my autodidact reading leads me to the idea that mosaic virus refers to many types of plant viruses, which have in common that they cause blotching and disfigurement of plant leaves.  For me, the interest is in figs.  However, mosaic viruses have big effects on roses, tobacco, and many other plants.  Possibly, many if not most plants in commercial horticulture.

Fig mosaic disease

For figs, it is thought that ALL fig trees grown in the US, if not the world, are infected with various types of FMD (Fig Mosaic Disease).  They grow and produce fine anyway.  But the virus might weaken the plants, result in lower production, and less vigor.

Rose mosaic disease

Much of the research on plant disease is with tobacco, because of the big $$$$ available and at risk.   So there is a fair amount of info about tobacco mosaic disease.  Whether that info can apply to other viruses that cause plant mosaicism, or other plant species, depends on the plant and the virus.

Tobacco mosaic disease on tomato leaves - why you should not smoke or use tobacco products around tomatoes

It turns out that plants DO have immune systems.  They work very differently from animal immune systems.  This is via modern, respected scientific research.    I don't mean to put down folklore, which is collected wisdom and observation through generations of farmers and gardeners, but I need the role of science to understand why things work, and what works, and what doesn't.

Not getting into the molecular biology, but it turns out that the role of aspirin for plants, has nothing to do with the effects of aspirin on animals.  Plants produce aspirin, or the similar salicylic acid, to activate genetic responses to disease.

The research has a long way to go.  I don't know how it will pan out.  For horticulture, and agriculture, we propagate and breed plants for flavor, favorable growth characteristics, and novelty, but disease resistance is usually secondary.  Not always - Liberty Apple is an example of a disease resistant apple that came to modern orchards. But often.  Then we grow the plants in close proximity - fields, orchards, nurseries - creating ideal environment to spread disease.  Aphids and other biting insects carry disease from one plant to another.  So do pruning shears.  Proliferation of Rose Mosaic Disease is thought due to the vast majority of roses are grafted onto infected rootstock; the rootstock comes from cuttings passed down from generations of propagation from rootstock that was infected generations ago.  

Bottom line, modern practices likely reduce plant ability to mount immune response, and increase liklihood of infection.  Since the infection often resides silently (like herpes virus in humans), we often don't know by looking that it is there.

Back to aspirin.  In the Cornell link, above. and a variety of other sources, there is evidence that aspirin water (approximately, one aspirin tablet dissolved per gallon of water), sprayed on leaves and allowed to soak in, can activate the plant immune system and fight plant disease.  Quoting the Cornell article (using the tobacco plant model), 

"When tobacco mosaic virus attacks a tobacco plant....the immediate visible effect of SABP2 is to enable salicylic acid to induce the so-called hypersensitive resistance response. "We see programmed cell death at the site of the attack as plant cells sacrifice themselves for the overall survival of the plant,"...helps restrict the infection to a small part of the plant. Something similar happens in animal systems, when virus-infected cells or cells with defective growth control that could become cancerous undergo programmed cell death,"...  Even as the infection is being contained, the plant begins to signal other parts of itself that it is undergoing attack. "This leads to long-lasting, broad-spectrum systemic resistance to infections against the initial attacking pathogen and also against other viral, bacterial and fungal pathogens," Klessig says. "Systemic acquired resistance can last throughout most of the life of an annual plant."

There is a fair amount of folklore, or hobbyist, interest in use of aspirin water for plants.  Some hobbyist with interest in a particular plant -

Have advocated spraying their plant collections with aspirin water.  (I have no interest in growing that plant, but those advocates also pioneer use of other plant techniques, such as mycorrhizal inoculant and plant hormones for propagation).  Also, however, there is interest from a variety of sources 

In addition to viruses, aspirin could have roles for plant infections with bacteria and fungi.  I have a particular issue with peach leaf curl - a big problem with peaches in Pacific Northwest, my trees last year, here:

So, I am experimenting with aspirin water.  It's made by crushing a commercial aspirin tablet, dissolve in a cup of hot water, then dilute in room temperature water to one gallon.  That gets poured into a sprayer, and sprayed on the leaves.  Whether it helps or not, it helps me feel like I'm doing something.  It's sprayed on the leaves, not poured into the soil.

on ehow, claims of increased production for vegetables, using aspirin water.  From that link:  "Spray your garden vegetable beds every three weeks with a solution of 2 gallons of water and 1.5 ground aspirin for healthier, larger vegetables. According to the National Gardening Association, the University of Rhode Island Organic Vegetable Garden in Kingston, feeding a garden water containing aspirin in this ratio and frequency increased "yields and the quality of tomatoes, eggplant, basil, and other vegetables.
"


Long post.  Hope it's informative.

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It is! Thanks, Sentient!

I may try it on some of my plants that have problems.

I guess that settles that issue!

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