Pests, Pathogens, and too Little Phosphorus

Before planting, California lettuce fields are often chemically fumigated to get rid of Nematodes. Chuck and I wish that the public were made aware of the major differences between buying field lettuce and our purer, crisper, hydroponic varieties.

In addition to Nematodes, lettuce grown in soil is prone to a number of other major pests. Black Cutworms, Beet Armyworms, the Cabbage Looper Caterpillar, and the Corn Earworm plague field-grown lettuce crops.

Aphids, Mites, Thrips, and Whiteflies can also infest greenhouse-grown lettuce, but only if common sense sanitary procedures are ignored. As well, Chuck and I make sure that our four greenhouses are equipped with fine-mesh screens at every opening, to make sure that flying insects do not invade our bread and butter crops.

Two species of Aphid especially like to munch on lettuce. The Potato Aphid (Macrosiphum euphorbiae) and the Green Peach Aphid (Myzus persicae) feed on plant sap found in lettuce leaves and their feeding habits will certainly reduce plant vigor.

These Aphids may vector the Lettuce Mosaic Virus (LMV) and introduce toxins into plant tissue which may necrotize the tissue in that particular area. Aphids also excrete a honey-dew-like substance that attracts ants to soil-grown plants.

Since both Chuck and I are committed to Integrated Pest Management (IPM)—meaning that we choose to exhaust every natural method of disease and pest prevention, before we would ever consider the use of chemicals to fight or prevent infestations—we prefer prevention to allowing a problem to invade our greenhouses.

Barricade is usually the first product to go into our Nutrient Pond pre-mix tank, since it takes a while to dissolve. Many growers mix it in hot water and shake it vigorously, and still pour it in the night before so it can blend into the other ingredients of our nutrient solution.

Barricade is a potassium silicate product which fortifies the cell walls of our lettuce, pak choi, and herbs. Stronger and thicker cell walls are able to withstand potential assaults by insects or pathogens much better than unfortified cell walls.

Another product that we use both as a foliar spray and a root zone additive is Scorpion Juice. This Advanced Nutrients formula imparts an induced systemic resistance to all our plants which is akin to giving them an immunization prior to any harmful microorganisms or parasitical insects making contact with our produce.

One year the Aphid infestation was so great in our original greenhouse, the one that grows Boston Lettuce, that we purchased a quantity of lady beetles and tiny wasp parasitoids to help us get rid of the Aphids. Natural predators work much better in a greenhouse setting rather than out in the field, where they can scatter and thus be ineffective.

Any kinds of pests or diseases have to be dealt with prior to harvest. I never forget biting into an apple when I was a child and being greeted by a cheerful green worm coming out of the apple’s core. It was an organic apple, no doubt, but allowing a parasite or a malady to proceed with your produce to your customers is a sure-fire way to lose them.

A word about Protector is appropriate here. This AN product was reintroduced recently after sales of it were stopped for a time, while the company negotiated with the government whether or not it was a fungicide.

Designed to fight and prevent Powdery Mildew, Protector is a highly effective product but it is not a chemical fungicide. So now it’s back on the market and it is very useful to have in our arsenal, especially during the potentially high humidity of the summer months.

Protector is accompanied by a word of caution. For flowering plants, do not spray after week 2 of bloom. For plants that will be consumed, such as our lettuce, pak choi, and herbs, do not spray within a few weeks of harvest.

It seems that Powdery Mildew needs a specific pH range to thrive and survive. The potassium bicarbonate in Protector changes the pH of the surface of your plants, making it an inhospitable place for this insidious fungus to grow.

However, the same properties that cause the pH change could also alter the colors of your flowers or impart an odd taste to your consumables. So be forewarned and use Protector wisely.

I recently came across an eye-opening statistic. A pound of lettuce contains 95% water, 56 calories, 3.9 grams of protein, 0.3 grams of fat, 86 milligrams of Calcium, 2.2 milligrams of Iron, 1,420 milligrams of Vitamin A, and 54 milligrams of Ascorbic Acid, otherwise known as Vitamin C.

Optimum lettuce growth occurs between 60 and 70º F (15 to 20º C) but the night temperature has to be kept much cooler in order to ensure the production of good quality lettuce.

Another trick of making sure that the lettuce you grow is of good quality is feeding your crop the best fertilizer available in just the right quantity. Studies have shown that using too little Phosphorus will reduce lettuce yields, for instance.

Using too much fertilizer—feeding your lettuce too often—may result in a soluble salt buildup. This could have phytotoxic effects on plant growth, you could pollute the groundwater in your area with your discharge, and your budget will suffer since you’re paying for unnecessary plant food.

By using a scientifically measured amount of plant nutrients, clearly explained and demonstrated by the Advanced Nutrients Nutrient Calculator, my partner and I avoid the obvious pitfalls of too much or too little. Our basic fert is the AN 3-part, Micro, Grow, and Bloom, only without the Bloom for obvious reasons.

Lettuce has low salt tolerance. Soluble salt injury is manifest in poor germination and the diminished size of the harvested heads. We try not to go above 1.71 EC in our parts per million count, because if the electrical conductivity of the suspended particles exceeds that rate, yield losses can occur.

So when we mix in our supplements, additives, and root colonizers (at half-strength only!) we make sure that the PPM doesn’t exceed 1200 in each batch. We had to recalculate the figures given by the Nute Calc, not just by eliminating the suggested quantities of Bloom each week, but also by slightly cutting back on the quantities of the other ingredients.

For pak choi and the herbs we can get away with slightly higher densities of dissolved solids, but lettuce prefers lighter meals. We’re constantly re-evaluating what we’re doing and the recent bolting episode opened our eyes to the importance of protecting our investment.

Growing outdoors, higher fertilizer rates are required during the cold, winter months, but in our environmentally controlled digitally monitored greenhouses we tend to keep the rate of feeding the same year-round. This uniformity of effort is reflected in the uniformly high quality of our produce, much to the delight of our regular customers.