Digger Spade, MG Plant Detective
At the end of August a client brought samples of leaves from tomato, bean and zucchini, all of them looking pretty unhealthy. We found a few two spotted mites (Tetranychus urticae Koch) on the squash leaves which also had transparent areas where the mites had sucked out the cell contents. The other leaves didn’t show signs of disease and there were no insects or signs of insect damage.
The best control for the mites is the predatory mite Typhlodromus occidentalis so we did some more checking and found one. So mite warfare had been declared and may eventually limit the spotted mite damage. Then there are 81 pesticides available to homeowners that control mites but they would kill both participants in the biological war. The client has to choose.
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| Twospotted spider mite adult females (E. beers) |
Now to the generally sickly aspect of his plants.
Where is the clients garden? In Bellingham.
Did these problems show up last year? There was no last year, this is a new garden.
Were any soil amendments used? The plot is a raised bed that was filled with three-way soil.
Was the soil under the bed cultivated before the bed was installed? No, the bed about a foot deep was built on lawn and filled with the manufactured soil.
Was fertilizer added? Yes an organic fertilizer was added when the transplants were put in.
How about watering? Done by hand as needed.
Did he check moisture level at the bottom of the bed? No.
We told the client that three-way mix is called top soil but isn’t. There is no standard recipe but generally the mix is equal parts of soil (which is unlikely to be a rich loam), sand and compost. The mixture is screened and thoroughly mixed. Often the compost will be high in carbon and low in nitrogen and will compete for available nitrogen with the plants. If the soil in the mix is subsoil it will not contribute much by way of nutrients and sand has none. We told the client of the study done at Hovander on composted cow manure which showed that when more than an inch or so was used to amend the soil, vegetable yield in the first year was decreased. In the following years the higher additions, having had time to complete decomposition, had a positive effect.
Part of the problem may be the water supply which the drought made difficult to manage. Too little or too much can create the kind of problems we were looking at, as can intermittent shortages and surpluses. The soil mix drains well and dries quickly so the plants could be stressed by a water shortage. And the lawn the bed was built on may not drain well so there could be a soggy layer at the bottom of the bed that would drown the roots as they reached it. Ideally the lawn should have been cultivated and at least the first layers of amendment mixed with the local soil.
So what can the client do? Though it is late in the season add fertilizer, soluble will give the quickest response. Eventually worms will integrate the lawn and the bed. If the client was able to double-dig it this fall, that would best though a lot of work. Next year a complete fertilizer with micronutrients would help and a keen eye out for insects too.
D.S. and Associates