Spinach Handbook

Crop Health

Disease
Our experiments were periodically impacted by disease, sometimes with severe damage.  The most common symptom was a rot in the plant crown.  The problem would usually become apparent in some plants about 15 days after transplant, at plant age of 23 days.  We found a harvest of healthy plants was usually possible at a plant age of 20 to 25 days.  At that time the plant is undergoing maximum growth rate and a drastic loss of yield occurs with this early harvest.  If the crop can be maintained until plant age of 33 days, the yield is much larger.  This trade off should be seriously considered by prospective producers.  Some of the cultural techniques required to produce an adequate yield are different than those we used and will have to be developed by the producers.

We did not develop a solution to the rather severe disease problem, but some of our results indicated the problems are not completely intractable.  A summary of our findings about the disease situation is presented herein.

In our first experiments tap water was used in nutrient solutions.  Severe root damage occurred shortly after transplant.  Analysis found up to 4 ppm of Total Chlorine Residuals (TCR).  Further study showed that TCR of 1 ppm was toxic.  In a literature search, Date, et al. (1955) showed root damage resulted from .05 ppm chloramines.  A change in procedures incorporating extensive rinsing of the physical system with RO water and the use of RO water in nutrient formulations greatly reduced the root damage.

A rot appeared in the plant crown at various ages.  Several remedies were tried, including the biofungicide Streptomyces griseoviridis, the parasitic fungus Trichoderma harzianum, the bacterium Enterobacter cloacae and potassium silicate.  Unfortunately, none of these remedies proved to be consistently successful.  Applications of Streptomyces griseoviridis showed the most promise.

Some general rules which are applicable to disease reduction in all greenhouses are as follows:

• Do not bring other plant material or soil into the greenhouse.  This material may contain pests and pathogens likely to infect your crop.

• Keep the solution tanks shaded in some manner.  Algae flourish in wet, well-lit locations, and the solution tank is ideal for algal growth.  Shading the tanks, input and output pipes, and other "wet" equipment inhibits algal growth.  The algae will not harm the crop directly, but may act to weaken the crop to potential disease.

Pests
Pests in hydroponic production have not been a major problem.  Fast plant growth rates make pest population establishment difficult.  With continuous crop production, pest populations may have the opportunity to establish themselves.  Precautions can be taken to exclude pests from the facility, such as screening potential entry points (fans).  Few pesticides have been labeled for use on greenhouse vegetables.  Biological insect control is a viable alternative.