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Home  > Question of the Week

Question of the Week

Permalink I’ve been getting differences in Brix readings depending on how I extract liquid from my sample leaves. What would you consider the most accurate readings of leaf sap?

A: Brix is a useful tool when standards are known, but, according to Ohio State University Extension researchers, "At best, Brix values also appear to be indirect estimates of the general health of some crops and their potential sensory appeal to people. However, there is little scientific evidence to support the use of Brix values as anything more than an indication of the soluble solids content—and potential sweetness—of fruit and vegetable samples." The full fact sheet, Using Brix as an Indicator of Vegetable Quality, is available at

There is another school of thought that considers Brix readings to be indicative of much more, but if there are not standards for a given crop and a specific protocol for taking plant samples and readings, this becomes meaningless very quickly. For instance, foliar samples Brix readings can vary by the time of day, by the age of the leaves, by the season, by the means of expressing liquid, etc. If you establish your own protocols and are consistent with their applications, you might be able to determine something meaningful besides just the relative sweetness of the leaf juice or fruit juice. But if everything isn't well controlled and standardized, you will not know what you're really testing.

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Permalink How can I manage fungus gnats in my greenhouses?

Answer: Adult fungus gnats don't damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in seedlings and young plants. Significant root damage and even plant death have been observed in interior plantscapes and in houseplants when high populations were associated with moist, organically rich soil.

Fungus gnats can be managed by decreasing food and habitat. Fungus gnats feed on fungi, decaying organic matter, and plant roots, particularly in very moist environments, so reduction of the fungus food source must be integrated into a program of managing other food sources and breeding habitat (see Management, below). Note that fungus gnats are not the only type of flies that may exist in a potted plant or greenhouse environment. Other families of flies include moth flies (Psychoidea), fruit flies (Drosophilidae), and shore flies (Ephydridae), not to mention various species of predatory flies and wasps (1). Proper management of any pest requires accurate identification of the pest. Once the pest and its life cycle are known, then management strategies can be developed around "weak links" in the life cycle.

Fungus gnats and shore flies are known to transport Pythium spores and innoculum of several important plant-pathogenic fungi around the greenhouse. Larvae feed on decaying organic matter as well as small roots and have recently been shown to act as vectors for pathogenic fungi such as Botrytis and Fusarium (2) as well as Phytophthora (1).

Cultural controls are the easiest and most practical way of managing (and preventing) fungus gnat populations. Soils (or woodchips) that are constantly moist and have undecomposed organic matter make ideal habitats for fungus gnats. Unfortunately, many potted plants or mushroom-rearing facilities—particularly those with bark or wood chip mulch—present ideal environments for these pests. Avoiding overwatering and allowing the soil to dry to the greatest extent possible short of harming the plant is one option for managing this pest. Planting pots should be well drained, and standing water should not be allowed to collect in the catch dish under the pot. Adult populations may be monitored using yellow sticky traps placed (sticky side up) on the soil/mulch surface of the potted plant. Another approach to prevent initial infestations is to pasteurize potting soil prior to planting.

In 1985, Dr. Dick Lindquist of Ohio State University showed that fungus gnat problems are most serious in potting mixes amended with composts lacking in maturity (not completely composted). Microbial activity is excessively high in such mixes, and fungus gnats thrive.

Another consideration is having continuous production cycles, which allows for infestation of new potting material by previous generations of the fungus gnat. It may be worthwhile to try to break the re-infestation cycle by not allowing access to potting soil in order to rid the greenhouse area of adults.

Other management options include use of neem-based formulations, use of non-toxic bacterial formulations, such as Bacillus thuringiensis H-14 (Gnatrol), insect-attacking nematodes (mostly Steinernema feltiae), and predatory mites in the genus Hypoaspis (2, 3).

Hypoaspis miles (Predatory mite)
This predatory mite prefers to feed on first instar fungus gnat larvae and will also feed on thrips pupae. It may also feed on debris and algae. It is important to make releases early in the growing season before fungus gnat larval populations are abundant. Applications can also be directed to the soil beneath greenhouse benches. Avoid applications into the growing media prior to planting because this decreases survival. Applications need to be initiated after planting and the growing medium should be moist but not saturated. Hypoaspis miles is active when growing medium temperatures are greater than 50 degrees F.

Steinernema feltiae (Predatory nematode) (3)
This beneficial nematode attacks fungus gnat larvae. Nematodes are applied as a drench to containers or flats and they can also be applied through drip-irrigation systems providing that filters are removed. Apply nematodes two to three days after inserting cuttings, planting plugs, or starting seeds. To assess the viability of shipments prior to application, place a small quantity of the product in a shallow container with a few drops of tepid water. After a few minutes, look for active nematodes which have a slight 'J' curve at the ends of their bodies.

Repeat applications are usually needed. Growing medium temperatures must be 50 to 80 degrees F with optimum temperatures of 60 to 70 degrees F. Irrigate the growing medium before and after applying nematodes. The nematodes require moisture in order to move within the pores of the growing medium. Apply nematodes in the evening or on cloudy days because they are extremely sensitive to ultraviolet light desiccation. In general, beneficial nematodes are compatible with most pest control materials except for carbamate and organophosphate pesticides.

Coenosia attenuata (Hunter flies)
This is a predatory fly, native to southern Europe, that was first found in a greenhouse in upstate New York, and has subsequently spread across the United States. Hunter fly adults resemble a half-size version (or smaller) of the common house fly adults. In addition to fungus gnat adults, hunter fly adults attack and feed on shore fly, whitefly, and leafminer adults. Adult hunter flies only attack prey that are flying. The soil-dwelling larvae are also predaceous and feed on fungus gnat larvae and other insects in the growing medium.

For information on materials that can be used to control gnat populations, see ATTRA's Biorationals: Ecological Pest Management Database, available at

1) Osborne, L.S. and W. Fooshee. 1998. Fungus gnats. University of Florida, Central Florida Research and Education Center.
2) Pundt, L. Managing Fungus Gnats and Shore Flies in the Greenhouse. University of Connecticut.
3) Smith, Tina. 2012. Fungus Gnats and Shoreflies. University of Massachusetts Amherst, Greenhouse and Floriculture Program.

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Permalink Would using seed from a friend’s stand of "superior" black walnut clones result in a stand as good as the original clones?

Answer: You will always have genetic variability from seedlings. Planting seed from known (female) parentage is good, but you don't really know the male parent since it could be pollen coming from nearby woods or it could be from within your friend's planting. Now, if he planted all his land to one clone, it's almost certain that the male parent is coming from the woods because, though walnuts are potentially self-pollinating (there are no barriers in the flower preventing it), in actuality there is "asynchrony" of male and female flowers (males, called catkins, bloom at different times than female flowers) which is one of Nature's way of preventing inbreeding.

And even if your friend had more than one clone in his planting, and thus you could be more certain that there was enough overlap of male and female bloom time within the planting, there would still be chance genetic recombination at the gamete level, and you could still be getting some pollen blowing in from the woods.

The upshot is that you will never know for certain what the genetics are going to be for the trees coming from the seed of your friend's walnuts. Even if there were no walnuts in the woods nearby, there would still be genetic recombination going on, and it's practically impossible to predict what the resulting progeny will look like.

So, now you have to consider what your long-term goals are because this is a very long-term investment. If you were to discover 10 years or more from now that you had a lot of genetic variability in your planting and some inferior genotypes, how would you feel? That situation might be fine with you. Genetic variability is usually a good thing and might have benefits (like disease resistance) that we can't see from this point in time. But if you're hoping to have a stand that is more or less uniform and that you can treat uniformly, including harvesting, you might regret it.

You're probably going to be harvesting these trees in your old age, if you're lucky. These will probably be for your kids and grandkids. Looked at that way, the right investment at the front end (if it's not just prohibitively expensive) begins to look better with time. If, on the other hand, you simply don't have that kind of money to invest right now, you would still probably get superior (compared to random seedlings) trees by using the nuts from your friend's place.

For more information on tree crop production, see Tree Fruits: Organic Production, available at

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Permalink I am interested in applying for organic certification. What will an organic certifier look for?

Answer: Three years of land-use records is critical, with brand names and formulations of each material used, its location, and date of use. Plans and examples of how the producer will do recordkeeping in the future is important, even if every detail has not been documented in the past. The USDA National Organic Program (NOP) requires that records be kept for five years going forward.

An organic inspector will look at what is happening on the ground in the operation, and that key things are documented and recorded. Organic inspectors are more concerned with your records being accurately and faithfully kept, regardless of whether they are hand-written or saved on a computer. The key components of records are:

a) What goes into a farm: Receipts for material inputs with full brand name and formulation (so there is no question about being able to verify their allowability). It is easier to review these when there is one folder for seed and planting stock, with clear notations about organic and untreated seed, and any allowed treatments, and a separate folder for fertilizer or pest-management inputs.

b) Practices carried out on the farm, such as material applications (including clear identification of materials, dates, rates, and crop on which it is used) and crop rotation (one simple but effective way to track this is directly on multiple copies of a farm map).

c) What comes out of the farm: Harvest and sales records may be one and the same for some operations. A Market Load List (a template can be found in the Organic Market Farm Documentation Forms publication referenced below) should show market location, date, product taken to market, and product actually sold, with the total dollar amount of sales. Notation of travel, stall fees, and related marketing expenses, while not required for certification, is important to assess the true costs of marketing. Having records organized by market and date makes it easier and faster to review them for adequacy and completeness. Receipt books or invoices should also track the quantity, date, and organic status of product sold, and to whom. Records can be organized any way that makes sense to you and your farming operation, and as NOP regulations state, track every transaction such that they may be "readily understood and audited."

For more information the organic certification process, see the following ATTRA publications:

• Organic Certification of Farms and Businesses Producing Agricultural Products
• Organic Certification Process
• Organic System Plans: Market Farms and Greenhouses
• Organic Market Farm Documentation Forms

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