Crop Protection and Seed Saving

Nematode management.

Plant-parasitic nematodes are a problem for farmers throughout the tropics and subtropics. Root-knot nematodes cause the most damage at ECHO. Information about several methods of nematode management is in EDN 75-1. Also see AZ 201; AZ 102.

Seed inoculants to maximize the ability of leguminous plants to fix nitrogen. 

Leguminous (nitrogen-fixing) crops and trees are an excellent source of non-chemical nitrogen for agricultural systems. Examples include crops like beans, cowpeas, and pigeon pea, or trees such as acacia, leucaena, and sesbania. The biomass of many legumes can add over 100 lbs/acre (112 kg/ha) of nitrogen every year. Leguminous plants “fix” nitrogen (take nitrogen from the air and convert it to a form plants can use) with the help of special bacteria from the Rhizobiacae family that colonize and form nodules on plant roots.

If your legumes look chlorotic (yellowing of the older leaves, sometimes caused by a lack of nitrogen in the soil), it is possible they have not been effectively colonized by bacteria. Some legumes must be colonized by certain groups of rhizobia in order to effectively fix nitrogen, so it is important to find out which bacteria species is needed for a given legume. This is especially important if the legume you want to grow is not common or native to an area; the local rhizobia may not be as efficient as those present in soils where the plant came from. A fairly extensive list of inoculation groups appears in an old publication entitled “Legume Inoculation” (www.caf.wvu.edu/~forage/library/; scroll down to the section on “Legumes”).

Look for more information in an upcoming EDN article. Inoculants for major leguminous crops are inexpensive and readily available in the US (e.g. www.agstore.net)..) The ECHO seed bank is looking for international sources of inoculants to recommend, and we welcome your suggestions.

Bean weevils controlled by tumbling the beans.

AZ 270 featured an article summarizing a technique for controlling bruchid weevils in harvested beans. Researchers found that weevil larvae must brace themselves against a hard surface (side of a container or neighboring bean) in order to bore through the surface of a dried bean. They then determined that this activity could be disrupted by turning each sack of beans end-over-end two to three times per day until inspection revealed no remaining live adults. The tumbling action of turning over the sacks dislodged the weevil larvae before they could tunnel through the seed coat, forcing them to begin boring into a different bean. The next tumbling time forced the weevils to begin on a third bean, etc. Weevils die if they cannot get to the food source inside the bean in a little over one day, so the containers that were tumbled had 97 to 98% fewer weevil-infected beans than those in non-tumbled containers. This technique would likely be successful with other pests with similar biology.

Controlling seed storage conditions.

In many cases, seeds of high-value crops must be stored for a period of time before planting. During that time, proper storage conditions are absolutely critical, as seed quality can deteriorate rapidly under high heat and humidity. Quality of stored seeds depends on length of time in storage, storage temperature, and moisture content. With the exception of “recalcitrant” seeds (e.g. some tropical fruits like mango, jackfruit, and avocado that only remain alive for a few weeks or months), seeds of most crops can be stored for several years if the sum of the storage temperature (in ºF) and the percent humidity is close to 100. If seeds are stored at 80°F, for instance, the ideal humidity in the room or container should be around 20%. How can either temperature or humidity be reduced with no electricity? One technique we have tested involves storing seeds and oven-dried rice in a sealed container such as PVC pipe, and then burying the container in the ground. The rice absorbs moisture, keeping the relative humidity at 20 to 40%. Burying the container stabilizes the temperature. EDN 86-1.

Saving your own vegetable seeds.

Saving seeds of vegetable crops could be of great value where there is no regular supply of quality seed. An article in a July 2006 issue of Avant Gardener is summarized in EDN 94-1. It addresses seed production issues such as the maintenance of seed purity (making sure that varieties do not cross with each other) and genetic vigor. It also provides helpful hints for growing, collecting and saving various types of vegetable seed, such as the recommended distance between plants of the same variety; how various types of vegetables are pollinated; and the minimum number of plants of a particular vegetable that must be grown and harvested in order to maintain adequate genetic vigor. Those new to seed saving will probably want to start with a self-pollinating crop such as beans or tomatoes. Seed purity is easier to maintain with selfthan with crosspollinated crops, and one does not need to harvest seed from as many plants.

Neem leaf tea or neem seed oil to discourage insect feeding.

A compound called azadirachtin, present in neem leaves and seeds, is known for its usefulness in controlling insect pests. Neem seeds contain greater amounts of azadirachtin than the leaves, but the trees do not produce seed year round. EDN 54-3 recorded observations by Cindy Fake in Mozambique who used the leaves to make an insecticidal tea. The tea was made by pounding 500 g of green neem leaves in a mortar and pestle, adding 10 liters of water, letting the mixture sit overnight, and then straining it through a cloth with a small amount of soap. The soap was used to help the resulting spray stick to the leaves. Backpack sprayers were used to apply the tea to one of four hectares of maize and cowpea. Red locusts attacked surrounding plots of non-treated maize and cowpea. They landed on the neem-treated crops but did not feed on them. See ECHO’s “Neem” TN on making an even more potent insect spray from neem seed oil. AZ 200.

Tephrosia (Tephrosia vogelii), a valuable green manure with insecticidal properties.

Beth Doerr, a current staff member at ECHO, found in Malawi that tephrosia produced abundant leaf matter that could be used as a green manure or insecticide. According to observations by Beth and an extensionist in Tanzania, tephrosia leaves were found to be effective in several applications: 1) combining a tephrosia leaf extract with soap mixture to control aphids on okra; 2) using dried leaves for insect control in stored grain; 3) controlling insect pests on animals (cattle, sheep, goats) and 4) applying a leaf extract/soap mixture to the walls of a room to repel mosquitoes. Caution: tephrosia leaves have been used to kill fish, but this is not recommended as there have been reports of ill effects on people eating poisoned fish. More information on this subject can be found in AZ 203 (insecticide, etc) and EDN 65-7 (fallow management).

“Short day” onions to produce bulbs in the tropics.

In tropical and subtropical latitudes, only certain varieties of onions, called “short day” onions, will produce bulbs. If you buy “long day” onion seed that is bred for more northern climates, your onions will produce tasty green salad onions. They may thicken a little, but will not produce bulbs. There are good short day varieties available from many suppliers. Onion scientist Dr. Lesley Currah tells ECHO, “A well-organized seed catalog will not just say whether onions are “short” or “long” day but will list the ideal day lengths for each variety, e.g. 11 to 13 hours, 12 to 14 hours, etc. AZ 62.

Storage of bulb onions.

Growing bulb onions can be profitable—if the farmer and/or the retailer can store them. Tom Post wrote to ECHO about how an innovative local farmer’s success growing bulb onions turned into failure. The farmer found a way to produce nice bulbs in a region where few farmers could achieve this. But the next year he could hardly sell the onions because the storekeepers who bought his onions the year before had suffered huge losses during storage. Thus, it is important to realize that cultivars vary considerably in how well they store. 

Cultivars suitable for storage should produce a number of outer dry scales or skins that form a vapor barrier around the bulb, thereby minimizing moisture loss and the entry of fungi or bacteria. Locally adapted onion varieties, selected over many years within the tropics, will probably store better than the imported types, especially the “short-day” varieties from temperate climates.

Bulbs may not store well if you add too much nitrogen fertilizer and irrigation that promote excessive bulb growth. For storage, the optimum relative humidity range is from 65 to 75%. For the farmer or villager without refrigeration, temperatures ranging from 25 to 30°C (77 to 86°F) would be the best choice. EDN 59-4 has several more tips.

Using owls to control rats

. It is estimated that each year, Malaysian oil palm growers and rice farmers spend millions of dollars controlling rats. Millions more dollars are lost due to the damage rats cause to crops. In the 1980s successful efforts were made by the Oil Palm Research Institute in Malaysia to increase the number of barn owls. All they needed to do was to provide the birds with artificial nesting sites. Inadequate nesting areas had discouraged owls from breeding despite the food surplus. The occupancy rate of nesting sites in one oil palm estate reached 80% during the breeding season. The Institute recommends placing one nest box every 10 ha.

An article in Groundcover (No. 24, 1996) concluded, “The Malaysian research has helped dispel the myth that barn owls hunt only in open areas. They found that the birds change their hunting mode to suit the vegetation. Instead of flying over the area scouting for prey, the owls perch on palm fronds and wait for rats to pass by. The researchers put up perching posts to encourage this.“ The owls eat almost nothing but rodents. A breeding pair of owls with offspring needs about 1200 rats per year. To encourage populations of owls and other natural predators (e. g. chameleons and snakes), islands of natural vegetation in fields should be preserved. ECHO has simple plans for a barn owl nesting box on our website. EDN 54.

For another idea for non-chemical rat control, including a helpful discussion of rodent breeding biology and its connection to the growth of a rice crop, see EDN 93-3.