Pests & Diseases

Like weeds, pests and diseases are simply organisms that are pursuing their lives in a way that is inconvenient for us.  That may sound awfully dry, but it’s true.  As mentioned previously, there are more microbes in a double handful of soil than there are people on this earth, and yet most of the time our gardens thrive; obviously only a few of them, on fairly rare occasions, cause us any disease troubles.  When you consider that there are something like a million insect species on earth, the fact that a few thousand of them have become pests (to us) seems like a miracle more than a plague.  Unfortunately, this statistic can’t keep the blight off your tomato plants or stop the leaf miners from tunneling in you spinach.  There are, however, some basic practices that can help keep pest and disease problems to a minimum—without resorting to dangerous pesticides.  Specific methods of combating particular pest and disease problems can be found in the Crop Guides.

Your most effective defense against both pests and diseases is brain power.  Get to know the problems, then figure out solutions.  Observe how and when the problems begin; that requires that you know the garden, the soil, the plants, the weather, all the variables that make your garden the challenging and wonderful gestalt that it is.  Careful observation will almost always provide a clue to control.

Let’s consider cabbage root maggot.  Without some sort of control, spring plantings of cabbage, broccoli, and cauliflower, as well as early turnips and radishes, are frequently ruined by these small white maggots.

My grandfather found a solution to this pest in his garden by focusing on its parent: a fly that lays its eggs in the ground near the stem of any one or all of the vegetables in the Crucifer family.  The eggs hatch into maggots which promptly wiggle their way into the stems of the plants in question.  If the infestation is severe the maggots will kill the plant.

To combat them, he sprinkled wood ashes around where the stem emerges from the ground was covered.

The case of the cabbage root maggot provides an excellent example of just how many different ways a particular problem can be solved.  The first step in controlling any pest, whether an insect, a mole, or the neighbor’s dog, is to get to know its life cycle, and then find a way to interrupt that cycle so your garden is no longer affected.

One of the first ways is to alter the conditions in the garden so they no longer favor the pest.  When my grandfather spread wood ashes around the base of his broccoli plants, it seems he either made the site unattractive to the fly, or so altered the soil that the eggs didn’t hatch.  This could be because of the dryness of the ash, which would desiccate the eggs; or their caustic chemistry—wood ashes and water yields lye—which would kill the eggs, while at the time marginally raising the pH at the base of the plant and providing extra potassium for good root growth.  Either way, the wood ash trick is a simple and effective solution.

Another simple method of pest control is to time your plantings to avoid periods of heavy pest pressure.  This strategy, based on his observation that later plantings weren’t bothered, would mean holding off planting until the major spring hatch of cabbage flies is past.  I found in my research some years ago (when I had more broccoli than I did wood ash) that in the Northeast this fly finishes laying her eggs about two weeks before the frost-free date.  Unprotected plants set out in our garden before this date are almost always attacked by maggots; those set out after almost never.  The solution was clear:  wait two weeks, and no control was necessary.

A third approach is to physically prevent the pest from reaching the plant.  There is an old-time organic gardening trick to beat cabbage root maggots that works quite well:  make flat cardboard collars three inches or more in diameter and set them about the base of the broccoli plants.  The mechanism here is obvious:  the fly can’t reach the soil to lay her eggs.  Even easier is to put a floating row cover on the bed immediately after transplanting.  Not only will the fly be kept out, but so will flea beetles, and the plants will benefit from a milder microclimate under the cover.

Three other methods that work in some cases (but not this one) are physical destruction of the pest, that is, hand picking and killing them; physical avoidance by crop rotation, so that the plants are not where the pest expects to find them; and the use of predators, say, the introduction of ladybugs to prey on the eggs of leaf-eating insects.  A fourth, additional method is interplanting:  you can confuse or repel pests by mixing in plants unattractive to them with those they feed on, so they have a more difficult time finding their favorites.  I have not found this to be totally effective in preventing pest problems, but it will lessen the damage.

What is the alternative that we have avoided by taking the time to know the natural history and culture of our garden?  The standard chemical pesticide used for the control of cabbage root maggots is Diazinon, as insecticide first put on the market in 1952.  Diazinon is one of about a hundred organophosphate insecticides, the first of which were developed as part of the nerve gas research done by Nazi Germany in the years before and during World War II.  The normal methods of application are as a liquid soil drench, or in granular form, mixed with the soil around the base of the plant, much the same way Big Sam used wood ash.  Unfortunately, this chemical can be taken up and moved throughout the plant, though it is thought to be partially metabolized in the process.  These metabolites, however, may be even more potent as nerve agents than the Diazinon itself.  Evidence from incidents of organophosphate poisoning indicate that although humans have bodily enzymes that can break down Diazinon, sensitization may occur which will prevent this detoxification from occurring if exposure is repeated.  Also, there is direct experimental evidence that Diazinon exposure causes birth defect in birds.

Diazinon persists in the soil up to three months.  If surfactants—used in most pesticide formulations to enhance their penetration, but not required to be identified on the label—are present, persistence may be much longer, though soil pH, moisture content, and temperature also affect its degradation rate.  Diazinon is also mobile in the soil by leaching, though the presence of large amounts of organic matter can restrict both its effects and its mobility.

If all this seems needlessly complex, I agree, so let me summarize:  a man-made substance invented as a result of chemical warfare research can be used on the food you grow (and then eat) in place of wood ash to prevent infestations of cabbage root maggots.  Unlike wood ash, however, Diazinon is expensive to buy, dangerous to use or store, contributes nothing to the fertility of the soil, and may cause long-term nerve damage and birth defects.  That’s progress?

Oftentimes at our garden center, customers looking for a quick fix solution to some garden problem would say, after I explained the background, say, of cabbage root maggot infestation and how they could solve it, “I just don’t have the time to learn all that organic gardening; it’s too complicated.”  But look again at the discussion above; which approach is the complicated one?

Managing a garden may be a complex activity, but it is a rewarding one, and one that brings us closer to the natural world.  In contrast, the apparent simplicity and ease of use with the Diazinon treatment masks not only a great complexity, but a host of dangers both to us and our gardens; it quite possibly creates more problems than it solves, though we don’t know for sure.  And that uncertainty is its biggest problem, to my way of thinking.  The hidden chemical complexity, the smokescreen of unperformed safety experiments that the chemical salesmen and their regulatory agents hide behind, when the say a particular pesticide “has no known dangers,” is simply not open to our observation, or our analysis.  We have to depend on them, and they are in it for the money … our money.  Al we will every have if we use their products is ignorance or anxiety, not bliss nor a concrete knowledge of the problems and their solutions.

I have yet to figure out how we as a people came to believe that every new product must be progress, and why the burden of proof should fall on those who suspect a product may have dangers rather than on  those who created it for their own profit.


When discussing sick or weak plants it is essential to distinguish between disease and deficiency.  Without proper care your garden will not thrive, and if there is either too little or too much nutrient in the soil, the plants cannot be expected to remain healthy and produce well.  Following the practices in outlined in the Soil Maintenance and Garden Planning sections, the gardener should face few problems with fertility, but keep an eye on the plants nonetheless—we discussed the visual cues to major nutrient deficiencies in the Growing Seedlings section, and they are generally the same for older plants as they are for seedlings.

If the plants begin to show symptoms of nutrient deficiency, don’t hesitate to side-dress with some immediately available yet organically stable fertilizer like fish emulsion.  There are critical phases of every plant’s development, and if not given good care and ample nutrition at those times, they will be weakened, and weak plants are more susceptible to both pests and diseases.  As organic gardeners we work toward the goal of letting the soil feed the plants without our direct intervention, but in practice it takes time to build the soil to a high level of fertility.  Just don’t panic and jump on the chemical treadmill.  Plants fed synthetic fertilizers on soils devoid of organic matter may develop deficiencies in a wide range of minor nutrients, ranging from boron to zinc, but using an organic fertilizer or compost makes it very unlikely that such deficiencies will ever develop.

Diseases, on the other hand, involve specific infections, and have more diverse specific symptoms.  While details concerning the diseases various plants are susceptible to are discussed in the Crop Guides, some general notes on disease prevention are appropriate here.

Where do diseases come from?  Many are already in your garden among the billions of bacteria and fungi always in the soil; others are brought in from elsewhere on seed, on the roots of purchased plants, even in the soil clinging to the shoes of visitors.  Given susceptible plants and the right conditions, disease can spread rapidly.  Gardens are not sterile places nor should they be, so the best we can hope to do is to keep the plants in top condition, exclude unknown sources of disease, and try to control conditions that favor their spread.

To avoid seed-borne diseases, buy seed only from reputable dealers, and save you own seed only from healthy, vigorous plants; store seed in a cool, dry, dark place.  Also, if particular diseases are especially prevalent in your area, search out resistance varieties of the crops you want to grow.  Read catalog descriptions closely; specific resistances are almost always mentioned if they exist.  With tomatoes, for example, resistance to Verticillium wilt is noted by a capital “V” after the variety name; Fusarium wilt resistance by a capital “F”; and so on.  A major part of most vegetable breeding programs is developing disease resistance; take advantage of that fact and you’ll have fewer problems.

My grandfather bought field-grown onion plants by mail, but I can’t recommend that—the source of the plants concerns me, and some soil-borne diseases like pink root of onions could be unintended passengers.  Pests or their eggs can also be brought in on plants purchased at garden centers.  If you do buy plants, examine them carefully for any sign of disease or pests.  Flying insects can come in on their own, or may be blown in by the wind, bringing disease with them.  Many bacteria and fungal spores are also carried on the wind, and may lie dormant on plants or in the soil for long periods, until conditions are right for them to grow and reproduce.

One present, what these micro-invaders need to establish a foot-hold on your plants is moisture.  Humid, still air and standing water on the plant laves provide an excellent place for them to thrive.  A typical problem is that of early blight in tomatoes: first the bacteria is splashed by rain from the soil to the lower leaves of the plants; once established on the leaves, it can spread in subsequent rains from leaf to leaf.  You may spread t yourself if you work in the garden during wet weather, by brushing from plant to plant with wet hands and clothes.  Thus, irrigation can also work against you if it is ill-timed and helps the spread of bacteria.

In addition to buying clean seed, inspecting plants brought into the garden, timing irrigation properly, and staying out of the garden when it’s wet, you should practice basic garden sanitation:  don’t leave dead or diseased plants in the garden where they can serve as a breeding ground for diseases; and don’t put them on the compost pile unless you’re sure it will heat up properly and kill the spores and bacteria on the diseased plants.  If you aren’t going to take the time to build a proper “hot” pile, burn the plants instead; you can put the ashes on the pile.  Finally, practice the kind of crop rotation discussed in Chapter 6, so that any diseases that do get a foothold in your garden don’t persist.

 Pest Control Equipment

If you will be using dusts and sprays you will need the equipment to apply them, and safety equipment for yourself. As noted above, many of the pest controls that are considered organic because of their natural origin are nonetheless as toxic to humans as they are to the pests they are meant to attack: they are preferable to synthetics primarily because they break down quickly into benign materials, and thus have no long-term, cumulative side effects on the environment. You should always wear protective clothing while using them. A paper mask is usually sufficient, but I wear an actual respirator to be on the safe side. Gloves and goggles are not strictly necessary, but still . . . we organic gardeners are a cautious lot. For a discussion of pest control methods, see the individual vegetable entries in the Crop Guide section.

The best kind of duster to get is the kind that has a rotating blower to power the dust, some sort of hopper to hold the powder, and a relatively long, adjustable nozzle to direct the dust. If your garden is large, get one that has a shoulder strap, as holding the unit while rotating the crank will quickly tire your hands. The adjustable nozzle is important because you need to get the dust up under the plants’ leaves where the pests hide out, and unless you plan to get down on your hands and knees each time you dust, the extension tube on the nozzle duster comes in handy. For small gardens a trombone-type plunger duster will work, but you’ll wear out your arms trying to cover any substantial amount of space with one.

The same concerns apply to spray equipment. The little pump bottles see at the hardware store may be fine for a small garden, but if yours is bigger than about five hundred square feet, go for a backpack sprayer and save yourself a lot of aggravation. There are a number of good models on the market that allow you to pump with one arm and spray with the other. These sprayers work very well for foliar feeding (applying dilute liquid fertilizer to plant leaves) as well as pest control.

ShepherdPests & Diseases

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