Seed may receive improper care on the farm or in temporary storage and become heavily invaded by hidden storage fungi. Upon transfer to another storehouse, it may spoil more rapidly than really sound seed. Invasion by storage fungi may occur very shortly after harvest.

We have found wheat coming out of a country elevator, 2 weeks after harvest, to be much more heavily infected than when it went into the elevator. We have found some lots of grain going into supposedly long-term storage to be infected so heavily by storage fungi that they were on the verge of spoilage.

SEED INVADED BY STORAGE fungi is already deteriorated to some extent and is much more subject to spoilage in storage than is sound seed. The latter could endure unfavorable storage conditions for some time without any damage.

Seeds or grain heavily invaded by storage fungi and almost ready to spoil often receive the same grade and command the same price as other lots free of fungi. Ordinary inspection procedures for quality, grade, and price fail to detect this condition. The invasion of seeds by storage fungi sufficient to cause a large decrease in germination ordinarily is not detectable by inspection with the naked eye, or even with a microscope.

The knowledge and techniques required to determine whether a given lot of seed is really sound or whether it has been invaded by storage fungi are now available. The technique can be used by almost anyone. It involves culturing the seeds on agar media developed especially for the purpose.

The major factors that determine whether a given lot of seeds will be invaded by storage fungi sufficiently to cause damage are moisture content, temperature, duration of storage, insect infestation, and condition of the seed when placed in storage.

Damage is relatively uncommon in seeds stored dry and cool. It may be significant when the weather between harvest and planting is warm and humid or if other storage conditions are improper. The amount of damage that will occur is a function of time in relation to moisture content and temperature. The lower the moisture and temperature, the longer seed can be stored without damage. Furthermore, there is an interrelation between moisture and temperature, so that within limits a drop in one compensates for a rise in the other.

Emphasis on prevention is even more important for storage fungi than for insects. No suitable chemical treatment has been developed to prevent infection by storage fungi or to control the infection once it is established. None of the seed-treating fungicides used to prevent attack of fungi that cause damping-off or root rot in seedlings is of value against the storage fungi. Damage in storage can be avoided only by maintaining the moisture content or the temperature, or both, at a level so low the fungi cannot grow. Both should be kept low for safety, especially in long storage.

One can use the agar culture test to determine how many and what kinds of hidden storage fungi may be present when seed goes into storage. The test should be applied to samples taken at intervals during storage to learn whether fungi are developing and whether the seed is safe for continued storage or is in danger of spoilage.

THE RELATION between insects and fungi in stored grain has been studied at the University of Minnesota.

We have found that invasion by such insects as the weevils is almost always accompanied by fungi, which add to the damage done by the insects. As the insects develop, they increase the moisture content of the grain by adding metabolic water from their life processes. This makes a condition more favorable for growth of fungus. The moisture can migrate to adjacent portions of the bulk of grain and start fungus growth even beyond the focal point of insect infestation. Because fumigation will kill the insects but not the fungi, damage can continue even after fumigation.

MOISTURE CONTENT of seed and drying newly harvested high-moisture seed to a safe storage level are important in preventing damage by insects and fungi. Seed that is harvested dry or dried soon after harvest and is free of insects will remain sound, in good condition, and of high germination for years. Many samples of wheat and corn stored 4 to 6 years at a moisture content of 10 to 12 percent have germinated more than 90 percent and were as sound as when placed in storage.

The upper limit of moisture content that seeds can tolerate varies with the kind of seed, the temperature, and the duration of storage.

The upper limits generally considered safe for long-time storage under average conditions are 13 percent for beans, peas, and cereal grains, including corn; 12.5 percent for soybeans; 10.5 percent for flaxseed; and somewhat lower for most vegetable seeds and peanuts. Seed stocks of most of these crops are stored at lower levels than those indicated. That is why loss in germination caused by storage fungi is relatively uncommon.

Seeds stored at a uniform and low moisture content may not remain so especially in bulk storage where temperature differentials may cause moisture migration. In the fall and early winter, the top and sides of a storage bin cool off first. Warm, moist air rises through the center of the bin, and moisture condenses out as it reaches the cool seed at the top. When the seed warms up in the spring, the moist grain at the surface may be invaded rapidly and spoiled by fungi.

This transfer of moisture may be so great that some of the seed is damp enough to germinate when it gets warm in the spring. It is not true that seed has an urge to heat and germinate in the spring. Seed will germinate when it becomes wet and warm enough. It does not feel the urge of spring any more than a stack of hay or a bale of cotton, both of which will heat if they are moist enough for the fungi that cause heating.

Moisture migration is of minor importance in small bulks of seed or in bagged seed. The latter may take up moisture from the surrounding air in the warm, humid climate of the South or Southeast. Moisture pickup from the surrounding air is usually limited to the outer few inches in bulk storage. The relative humidity of the interstitial air in bulk storage is related more closely to the moisture content of the seed than to that of the surrounding air. That is why it is important to store seed dry initially. When the seed is dry enough, both the seed and the interstitial air are so dry the fungi will not grow.

The rice and granary weevils do not develop in seeds that contain less than 8 percent moisture and do not grow well when the moisture is less than 11 percent, unless the seed temperature is 85 to 90 .

The bran beetles, such as the saw-toothed grain beetle and flour beetles, can live on food almost devoid of moisture if the temperature is favorable. They do, however, grow and reproduce more rapidly as the moisture content goes up.

Measuring the moisture content of seed is important but not always easy. Electric moisture meters often are used to give a rapid determination. They do not measure moisture directly; they measure the electrical properties of seeds or grain as influenced by moisture content. They have some limitations, because other factors may affect the electrical properties of seeds, the meter must be calibrated properly, and the instrument may get out of adjustment. With some instruments, the measurement of the temperature or the amount of seed is a critical factor.

Electric moisture meters may give a reading as much as 1.5 percent above or below the results obtained by the oven-drying method. The error may be even greater in recently dried seeds, where the moisture content is lower in the outer layer of the seeds than in the interior. Here the reading may be 2 to 5 percent lower than by the oven method. These differences are important if the moisture content of the seeds is on the borderline of safe storage.

Sometimes lots of seed with different contents of moisture are blended to produce a mixture believed to have a moisture content safe for storage. It is assumed that if equal portions of 16-percent and 12-percent moisture content seed are blended, the mixture soon will have a moisture content of 14 percent. This is not so. The seeds with the original high moisture content will remain higher than the average, and those with the lower moisture content will remain drier than the average. Much of the seed may retain enough moisture to permit a fungus attack, figures in the record books notwithstanding.

Assuming that accurate moisture readings are obtained, there are still snares to avoid. Care may be taken to obtain an average sample of seed upon which to determine moisture content. It is on the basis of average moisture content that seed is bought, sold, and often stored. There may be, however, portions with enough moisture to permit fungus attack. There may be a difference of as much as 7 percent between different locations in a large storage bulk. Differences of 2 to 4 percent are common. An average figure of moisture content is obviously of no value in evaluating the risk of spoilage when there is such variation.

Even individual seeds from a supposedly uniform lot may differ by as much as 1 to 2 percent in moisture. The usual methods of measurement will not reveal such a condition, but it can be important in storing seeds.

Considering the importance of moisture and the possible errors in its measurement, it is not surprising that most of fungus damage occurs in stored seed with a moisture content higher than it is believed to be by those responsible for the storage.

COOL STORAGE of seed is important because of the relationships between temperature and moisture in their effect on the development of insects and fungi.

The optimum temperature for most seed-infesting insects is 80 to 85 . Temperatures above 95 are not favorable and below 70 development is retarded. Flour beetles do not reproduce below 65 . The granary and rice weevils do not reproduce below about 60 . Most of the stored-product insects cease feeding and become inactive between 40 and 50 . Some species of mites will reproduce at 40 or even lower, but only if the moisture content of the seed is above 12 percent.

The optimum temperature for most storage fungi is 85 to 95 . As with the insects, development is retarded below 70 , and most of them grow very slowly, if at all, below 50 . A few species, however, will grow slowly below 50 . They may gradually invade seeds during the winter. When the seed warms up in the spring, they will develop rapidly and cause extensive spoilage.