The Values of Fumigants

John M. Harvey, W. T. Pentzer.

An advantage of using gaseous fungicides is that they can be much more uniformly dispersed over the treated product than can a liquid or a dust. They also can be applied after packing and loading commodities into a storage room or other enclosed space.

The requirements of an ideal gas to control diseases are that it kill the disease organism but not injure the commodity, leave no residue that is harmful to health, not be explosive or inflammable or otherwise hazardous, be effective in low concentrations and short periods of treatment, be noncorrosive to equipment, and be moderate in cost and readily available.

Such an ideal gas has not been found, but gases with some of those properties are effective in reducing postharvest diseases of numerous commodities.

SULFUR DIOXIDE is used mainly as a fumigant to control decay in stored grapes. Some varieties of California grapes are stored for several months at 30 or 31 F. and 87 to 92 percent relative humidity and are fumigated periodically with sulfur dioxide.

A prevalent postharvest disease of grapes, gray mold rot, is caused by the fungus Botrytis cinerea. Some years it has caused 40 percent rot in grapes stored without fumigation. The organism, widespread in nature, attacks different fruits and vegetables. At first the fungus grows just under the surface of the berry and causes the skin to separate from the underlying tissues. The name "slip skin" is applied to this stage of decay. Later the whole berry softens. In the late stages of decay the berries are covered by a gray growth of mycelium and spores of the fungus, which form the typical gray mold rot. The fungus often grows from one berry to another to form a nest of moldy berries.

The amount of gray mold in storage grapes depends largely on the weather before harvest. Rains and long periods of humid weather favor the development of gray mold in the fruit. Although the organism grows best at about 77 F., it also can grow enough at 30 to cause serious decay.

The present techniques for applying sulfur dioxide to grapes were developed over a period of many years. A. J. Winkler and H. E. Jacob, of the California Agricultural Experiment Station at Davis, in 1925 published the results of a study in which they found that at concentrations of 2 to 3 percent sulfur dioxide is absorbed very rapidly. Warm grapes absorb sulfur dioxide faster than cold grapes, mature grapes absorb less sulfur dioxide than immature ones, and injured grapes absorb more sulfur dioxide than sound grapes.

Varieties of vinifera grapes differ greatly in their rate of absorption of sulfur dioxide and in the amounts of the gas that are injurious. In the United States extensive use of sulfur dioxide for grapes is restricted to California, as most varieties grown in the eastern part of the country are highly susceptible to injury by the fumigant.

The extent of the injury is not apparent until grapes are removed from storage and held under market conditions. The cap stems and stems of the grape are affected most. They become yellow or bleached a desirable "injury" because it keeps the stems from drying to their normal dark brown or black color, a condition which lessens the attractiveness of the bunch.

A more serious injury may occur on the berries, however. It becomes apparent on colored varieties as a bleaching of the tissue near the cap stem or any injured spot and on green varieties as a yellowing of the tissue. Those injured areas may become sunken, and the grapes often look dull and lifeless. Severely injured berries turn brown and flaccid and are unsalable. They have a sulfurous flavor. With modern procedures for fumigation with sulfur dioxide serious injury is seldom found in stored grapes.

The sulfur dioxide absorbed during treatment is quickly dissipated from the tissues of the grape. Malaga grapes that contained 16 parts per million of sulfur dioxide immediately after treatment have practically none after 5 days. Emperor grapes that contain 13 P. P. m. after treatment contain less than 1 p. p. m. after 14 days of storage at 32 F. in the sawdust lug pack.

Therefore protection from mold by fumigation with sulfur dioxide is only temporary additional treatments or retention of small concentrations of the fumigant is necessary to prevent later decay. Some mold spores survive fumigation and may cause infection after the effects of the gas wear off. Besides, the fumigation does not kill infections inside the berry but merely inhibits growth of the fungus.

Several precautions have to be taken. The gas is an irritant and a poison in the concentrations used for grapes. It can cause serious injury or death to animals and human beings, and care should be taken to avoid exposure.

Grapes are quite tolerant of the fumigant in the concentrations used, but some other commodities may be ruined by the gas. Other products therefore should be removed from storage rooms where grapes are being fumigated. Leakage to adjacent rooms that contain other produce should be avoided. Sulfur dioxide has a corrosive action on metal. Exposed metal surfaces should be covered with protective paints.

The gas is applied commonly to grapes in storage from weighed amounts of liquid sulfur dioxide in cylinders. The cylinders are heated in a water bath outside the storage room. The volatilized sulfur dioxide is piped into the room, where fans disperse it. The usual procedure is to gas grapes while warm with an initial concentration of 1.0 percent sulfur dioxide for 20 to 25 minutes. Then come applications every 7 or 10 days of about 0.25 percent concentrations of the gas for 20 to 25 minutes in the cold-storage rooms. The gas may be exhausted from the room through vents or by passing it through a water spray.

This is the way to calculate the amount of sulfur dioxide needed for 0.25 percent concentration: The total capacity of the room is calculated. From that figure is subtracted the space occupied by the grapes, which will be roughly equivalent to one-half cubic foot per lug, when air voids between grapes are taken into account. Since 1 pound of sulfur dioxide is equivalent to 5.5 cubic feet of gas at 32 F., the free space (cubic feet) in the room multiplied by 0.25 percent divided by 5.5 will give the pounds of gas needed. Or: Free space times (0.0025 divided by 5.5) equals the pounds of gas needed.

Sulfur dioxide also can be applied to grapes as sodium bisulfite, added to the packages. It breaks down slowly in storage and reacts with moisture to release small amounts of sulfur dioxide into the air a characteristic that makes the treatment particularly good in situations in which the fruit is held for a considerable period and cannot be repassed by the conventional method. Sodium bisulfite is used mostly for grapes packed in sawdust. Five grams of the powdered chemical are mixed with the amount of sawdust required to fill the spaces around the grapes packed in a chest or lug. The material should be distributed evenly. It should not be used if the grapes or sawdust are wet. It should not be used in conjunction with other means of sulfur dioxide refumigation in storage.

Commercial fumigation of grapes has been successful, but injury to the grapes has not been completely eliminated. Therefore the aim of research is to find the point at which minimum injury coincides with adequate control of decay even though a storage room may include several varieties that were harvested at different times and therefore vary in susceptibility to decay and injury. The recommended treatment must be broad enough to cover diverse lots of grapes and conditions.

NITROGEN TRICHLORIDE has been used chiefly as a fumigant for stored citrus fruit. It has been used sometimes on cantaloups, tomatoes, peppers, asparagus, and onions. It does not control decay adequately in grapes and causes an undesirable pitting and discoloration of the stems. It is formed by the reaction of chlorine with ammonium chloride.

It is highly explosive and toxic. To be used safely, it must be greatly diluted in air and generated in special equipment at the time of use. The gas is fed into storage rooms where fans mix it thoroughly with the atmosphere. Only trained persons should use it.

Long exposures to the gas or excessive concentrations may cause injury to citrus fruit. Unusually high concentrations may occur in parts of storage rooms that have poor circulation of air. The result is sunken and browned areas around wounds in the fruit.

Citrus fruits are subject to the attacks of numerous decay organisms during storage. Blue contact mold (Penicillium italicum) and green mold (P. digitatum) are the most common types. Stem-end rot of citrus caused by Phomopsis citri and Diplodia natalensis is serious in the Gulf States but of minor significance on fruit grown in California.

Many citrus fruits, if they are stored at temperatures near freezing, become subject to physiologic disorders. Fungicidal materials have to be used therefore to control rot-producing organisms.

In studies conducted at the Citrus Experiment Station at Riverside, Calif., L. J. Klotz found that nitrogen trichloride was effective in the control of blue mold and green mold rot of citrus.

Since the gas is effective only against spores on the surface of fruit and not against mycelium within the rind, he recommended that the fruit be gassed soon after picking, before decay starts. When oranges were to be stored for long periods, he recommended one to three exposures of 3 hours each to 8 milligrams of nitrogen trichloride per cubic foot at intervals of 3 to 4 days at the beginning of the storage period. Reinfection was prevented by subsequent weekly gassings. Decay of fruit stored 3 to 4 weeks was reduced 50 to 75 percent by treating with nitrogen trichloride. The recommended concentrations of nitrogen trichloride, the duration of the gassing treatment, and the interval of gassing vary with the tightness of the storage room and the ease with which the gas can be passed through the packed fruit. Specific treatments therefore must be determined for individual storage rooms.

Lemons are much more sensitive to nitrogen trichloride than oranges. Low concentrations of gas at frequent intervals will reduce the amount of decay. Storage rooms for lemons are gassed with concentrations of 1 to 4 milligrams of nitrogen trichloride per cubic foot for 4 hours 3 to 7 times a week, depending on the condition of the fruit.

A. Lloyd Ryall, of the Bureau of Plant Industry, Soils, and Agricultural Engineering, and G. H. Godfrey, of the Texas Agricultural Experiment Station, found that nitrogen trichloride is effective against stem-end rot as well as blue mold and green mold rot of Eureka lemons in Texas. Lemons treated with 0.003 parts per million of nitrogen trichloride for 4 hours at 48-hour intervals developed only 13 percent decay in 2 weeks. Untreated lemons developed 70 percent decay. Even greater control of decay was achieved if the lemons were dipped in a sodium metaborate solution before gassing with nitrogen trichloride.