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Environmental stTesses on plants, diseases, and insects
Wind, water, and insect borne spores from colonized debris, sclerotia, and soil sources
Whorl to silking
and Colonization period-
t H a r v e s t period 4 4 - S t o r a g e and -b
b F t susceptible
65 time of infection 85
Approximate no. of
damage to grain by insects
Increasing post-infection aflatoxin accumulation
figure 1 The chronology of corn kernel infection by Aspergi/lusJ?avrts and subsequent atlatoxin contamination. Source: Widstrom ( 1992).
N. W. WIDSTROM
standard recommendation for corn grown in the South and other warmtemperature locations, especially those that have sandy soil with low waterholding capacity (N. C. Aflatoxin Committee, 1977; Glover and Krenzer, 1980,
Smith, 1981). The risk for aflatoxin contamination of corn, however, seems
always greatest under drought conditions, regardless of soil type (Tuite et af.,
1984), and the recommendation made to avoid contamination is to alleviate stress
by irrigation during the reproductive period (Jones, 1983) or adjust the planting
date to move the critical period of grain filling to a period of minimum stress
(Widstrom ef al., 1990).
All irrigated corn production systems require some form of soil moisture
monitoring to determine when irrigation is needed (Lee, 1994). Providing water
to the crop in efficient amounts at the optimum time will often determine the
profit margin for production; therefore, good judgment and experience are required for wise decisions regarding the best time to irrigate. Most growers,
experienced or not, will rely on mechanical or electronic devices to determine
when such soil moisture levels are critically low and irrigation is needed. Tensiometers or similar devices give the most reliable soil moisture measurements
and can provide moisture availability at several soil depths, giving the grower
adequate information to make a good decision on when to irrigate. This information, along with up-to-date weather forecasts, will maximize water use efficienCY.
The soil water tension in centibars required to call for irrigation will vary with
plant stage, soil type, and the adequacy of the irrigation system (Lee, 1994).
Young plants can survive slightly lower levels of moisture in the soil before
irrigation is applied, while large amounts of water are needed at the critical
flowering and grain-filling stages. Water demands are so high during the critical
stages that some plant stress is seldom avoided, especially if temperatures are
high and rainfall is limited during these periods.
In general, irrigation is called for when 20- to 25-cm-depth tensiometer readings are at 20 centibars or greater. Sandy loam soils usually require 25-40 mm of
irrigation when the critical soil moisture tension is reached. Heavier soils can
handle slightly more and sandy soils slightly less because of a lower waterholding capacity for sandy soils. Moisture deficit is among the easiest plant
stress-inducing factors to adjust and probably the most important because it
significantly impacts other stresses, such as insect damage and disease expression. Sandy soils, subject to frequent moisture deficit, along with high night
temperatures and greater disease and insect pressure, are the principal reasons
why aflatoxin contamination of corn is chronic in the southern and southeastern
Numerous studies have investigated the influence of irrigation on aflatoxin
contamination (Fortnum and Manwiller, 1985; Payne er af., 1986; Jones, 1987;
McMillian et af., 1991; Smith and Riley, 1992). These studies, without excep-
THE AFLATOXIN PROBLEM WITH CORN GRAIN
tion, demonstrated a net beneficial effect when irrigation was available. The
benefit of irrigation cannot always be realized, however, because it is often not
practical for the grower. In fact, corn is most often produced without irrigation in
high-risk areas, since more than one-half of the corn acreage in the Southeast is
grown under nonirrigated conditions. Alternative control measures must therefore be made available to growers for whom irrigation is either impractical or
2. Fertilization and Plant Nutrition
Initial observations of an increased incidence of aflatoxin contamination in
preharvest corn grown under low fertility conditions were made by Anderson et
al. (1975). This study in Georgia and others have led to a general consensus that
nitrogen fertilization of corn will influence aflatoxin contamination of the crop
(McMillian et al., 1991), even as it influences most other plant traits. The sandy
coastal plain soils of the southeastern United States are naturally very low in the
highly soluble nitrogen that is critically needed for corn, a heavy user of this
element (Gurley, 1965). Since a recommendation of adequate fertility is critical
for obtaining good yields, no serious changes in the fertilization recommended
for corn production were necessary with regard to aflatoxin contamination (Georgia Extension Aflatoxin Committee, 1978). A word of caution resulted from
experiments by Wilson et al. (1989a) when they demonstrated that overfertilization with nitrogen can also increase the incidence of contamination. This effect
can again probably be attributed to increased stresses on the plant and is a
concern only for those who are attempting to obtain maximum yields by applying
high levels of nitrogen fertilizer.
Other fertilization studies have given similar results regarding the need for a
supply of adequate nitrogen for the corn plant (Glover and Krenzer, 1980). No
single experiment can be cited as conclusive proof of the influence of nitrogen on
Contamination, since many studies also include the testing of other confounding
factors (Jones, 1983; Jones and Duncan, 1981). Stresses induced by inadequate
nitrogen for good plant growth are clearly a significant contributor to the contamination process (Payne et al., 1989). Lillehoj (1983) reasoned that since stress is
so convincingly implicated, and inadequate fertilization does induce stress, we
must include fertilization in the aflatoxin contamination equation.
The nutritional status of the plant, other than that expressed by obvious deficiency symptoms and lack of vigor, has not been demonstrated to be closely
associated with contamination by aflatoxin. Most nutritional factors have a high
impact potential on yield and are normally addressed because of their close
relationship to capacity for production. Many nutritional problems occur because
of nutrient solubility that is related to pH of the soil solution. Adjustments in pH
are made by the application of lime, as previously described.
N. W. WIDSTROM
a. Deficiency Symptoms
Nutritional deficiencies can usually be avoided if the appropriate fertilizers are
applied in a timely manner based on soil tests. Weather or unusual edaphic
conditions may induce deficiency symptoms in the corn plant due to lack or
unavailability of essential nutrients. Unlike the symptoms of disease development and insect activity, symptoms of nutritional deficiency can often be remedied and the plant restored to a healthy condition, if soil pH is in an acceptable
range for corn growth and weather is not extreme.
Frequent field inspections (as often as twice weekly) will assist greatly in
identifying plant stress due to nutritional inadequacies. Books and pamphlets are
available which not only describe deficiency symptoms, but also give excellent
pictorial examples to assist in diagnosis (Aldrich et al., 1975). County agents are
familiar with these aids and are available to assist the grower with both the
diagnosis and the remedy. Prompt attention to deficiencies will increase production and avoid the plant stress which can predispose kernels to A. flavus infection
and aflatoxin contamination.
b. Tissue Sampling
The plant is already suffering from stress if one waits until deficiency symptoms appear. Whole plant or leaf analyses can be useful for anticipating nutritional problems if a systematic program of testing is used (Lee, 1994). This procedure is very useful after the whorl stage for systems where fertilizer can be
applied through the irrigation system. A standard range of acceptable values has
been established for the major elements and most minor elements at the various
stages of plant growth (Smith, 1990). When samples are taken on a regular
schedule, the nutritional needs of the plant can be accurately anticipated prior to
stress due to nutrient deficiency. Stresses on the plant, especially during the
critical grain-filling stage, are known to increase the risk of aflatoxin contamination, and a regimented system of tissue sampling can eliminate nutritional
stresses during that critical period.
3. Cultivation and Weed Control
Cultivation and weed control are sometimes thought of as being synonymous,
but for purposes of this discussion, cultivation includes all types of tillage and/or
disturbance of the soil. It seems that cultivation practices associated with any
crop tend to increase the incidence of Aspergillus spp. propagules in the soil.
When compared to virgin, undisturbed prairie soils which produced 0 propagules, soils under conventional tillage and a legume-grass rotation yielded 256
propaguleslgram of soil (Angle, 1987). The degradation of aflatoxin also varied
from one soil to another, in that a fertile silt loam soil was more efficient than a
silty clay loam soil at decomposing aflatoxin B , .
THE AFLATOXIN PROBLEM WITH CORN GRAIN
Cultivation practices used under different rotation systems have not been
shown to influence aflatoxin contamination of the corn crop (Smith, 1981), nor
have the practices of conventional till versus no-till. Presumably, all tillage
systems provide an adequate supply of inoculum for infection and aflatoxin
contamination when environmental conditions are favorable. One tillage practice
that has proven effective in reducing contamination is that of subsoiling. Subsoiling allows deeper root penetration and renders the plant less susceptible to stress
under drought conditions. The apparent benefit of subsoiling is accomplished by
buffering the plant against water stress (Payne el al., 1986). Subsoiling is apparently the only tillage practice proven beneficial in reducing aflatoxin contamination, although recommendations usually only refer to tillage as an influencing
factor (Jones, 1987).
A good program of weed control is a necessity for every successful corn
growing operation. Eliminating weeds will obviously reduce water usage and
assist in preventing water stress on the crop, reducing yield losses for dryland
corn. As a secondary effect, good weed control will also reduce contamination
by aflatoxin, and consequently recommendations for control of aflatoxin usually
include judicious control of weeds by chemical or other means (N. C. Aflatoxin
Committee, 1977; Glover and Krenzer, 1980). The importance of addressing
weed competition with the crop in an aflatoxin control program has not been
documented by any formal studies to this author's knowledge, but weed control
is still an obvious and necessary recommendation (Lillehoj, 1983). An investigation that compared three cultivation rates to control weeds found no significant
differences among the treatments for aflatoxin production in the preharvest crop
(Bilgrami et a / . , 1992). The extensiveness of' a weed infestation needed to
demonstrate an effect on contamination is, therefore, an academic question that
requires no answer in the practical arena.
4. Disease and Insect Involvement
Plant disease is normally manifested by unique symptoms and as a reduction in
plant vigor. As such, stress on the plant is increased and susceptibility to other
organisms is increased, including infection by Aspergillus spp. Numerous diseases are prevalent on corn, all of which have a significant impact on plant vigor,
stress, and susceptibility to invasion by fungi such as the Aspergilli. The most
critical of these diseases with respect to aflatoxin contamination would be those
affecting the ear, especially the ear rots. Although known to be a member of the
complex of fungi invading the corn ear, A . f l a w s was not considered to be a
seriously damaging ear-rot organism, probably because of its generally nonaggressive nature (Taubenhaus, 1920). Ear rots caused by other organisms such as
Helminthosporium have been long associated with the presence of Aspergillus
spp. and sometimes with aflatoxin contamination (Doupnik, 1972). Aspergillus
N. W. WIDSTROM
,flavus is often referred to as an ear-rot organism (Campbell et al., 1993), although now recognized as well for its more notorious reputation as an aflatoxin
producer (Campbell and White, 1994). Its presence in the ear-rot complex keeps
it available for vigorous activity when conditions favor its development over
other organisms. Competition among ear-invading organisms will be discussed
in a later section of this chapter. The control of ear rots, stalk rots, and leaf
diseases has been accomplished primarily through plant breeding since chemical
control is not practical, except when growing specialty corns, sweet corn, or
breeding nurseries. Applications of several different fungicides in an experimental situation have been ineffective in significantly reducing aflatoxin contamination (Lillehoj et al., 1984; Duncan et af., 1994). The breeding approach will
undoubtedly be necessary in ultimately dealing with the aflatoxin problem.
The Aspergilli have long been associated with insect invasion of the corn ear in
addition to being members of the fungal ear-rotting complex (Taubenhaus, 1920;
Koehler, 1942). The present-day focus on an insect involvement was begun when
Anderson et al. (1975) reported preharvest contamination by aflatoxin and its
association with insect damage. Sampling studies of harvested and stored corn
conducted by the ARS at Peoria, Illinois, also began to show an association of A.
j a v u s with insect-damaged corn (Fennel1 er al., 1975, 1977). The association of
A. j a v u s and insects was examined in several preharvest field studies (Widstrom
et al., 1975; LaPrade and Manwiller, 1977; McMillian et al., 1978; Lillehoj et
al., 1978a; Zuber and Lillehoj, 1979) and subsequently the relationships between
insects, their damage to ears, and aflatoxin contamination of the corn was clearly
demonstrated (Lillehoj et al., 1975b, 1978a).
The role of insects in the infection and contamination process has been reviewed extensively (Widstrom, 1979; McMillian, 1983, 1987; Barry 1987). In
general, it has been determined that insect damage to the ear is consistently
associated with increased sporulations of A . flavus on the ear and increased
aflatoxin contamination of the grain (McMillian et ul., 1985b). This concept
holds even though other factors may tend to interfere, such as frequent heavy
dews that may cause insect damage to increase (McMillian et al., 1985a) and the
presence of A. parasiticus that is more closely associated than A . flavus with soil
insects (Lillehoj et al., 1980d).
Several investigations were initiated to determine which insects were most
closely linked to the infection and aflatoxin-producing process (Widstrom et al.,
1975). They determined that when confined to ear-feeding, the European corn
borer contributed more to the contamination process than either corn earworm or
fall arniyworm. The corn earworm is the most frequent lepidopterous ear feeder
in the South, and McMillian et al. (1990) found in a 12-year study that A . juvus
contamination of the corn earworm moth may also be closely enough associated
with preharvest contamination to serve as an early warning system to predict
eventual grain contamination by aflatoxin. A series of studies by Guthrie et al.
THE AFLATOXIN PROBLEM WITH CORN GRAIN
(1981, 1982) and McMillian et al. (1988) established the European corn borer as
a viable contributor to contamination when it occurred as an ear feeder, and only
its leaf-feeding habits prevent it from being the dominant insect associated with
The maize weevil (Sirophilus zeamais Motschulsky) is of special interest with
respect to the aflatoxin contamination problem because it functions as both a
preharvest and storage insect. Initial reports suggested that it was of relatively
limited importance and was judged to be a very inefficient vector of A . fravus
(LaPrade and Manwiller, 1977). Subsequent studies by McMillian et al. (1980a)
demonstrated that the maize weevil can contribute significantly to increased A.
jluvus infection on corn ears by transporting spores and damaging kernels. Other
investigators later confirmed the maize weevil as being an effective vector of
spores and capable of increasing aflatoxin concentration in kernels by as much as
100 times in the presence of the fungus (Rodriguez er al., 1983; Barry et a l . ,
1985). Heat and moisture generated by weevil activity in stored corn can be the
primary support for A . flavus growth (Dix and All, 1987). A recent addition to
the list of vectors is sap beetles (Nitidulidae) that can become carriers of the
fungus when both are present in the ear (Lussenhop and Wicklow, 1990). Other
ear feeders may also be capable of vectoring the fungus, but are considered
unimportant because their frequency as ear invaders is very low.
a. Prophylactic Measures
There are precautions that can be taken before planting to protect the crop from
damage and stress that will occur if insects, diseases, other pests and weeds are
not controlled. Such measures are in addition to the buiit-in precautions taken by
seed companies that provide protection through seed treatment and inherent
resistance of their hybrids to some pests and diseases. The use of prophylactic
measures often hinges on the growers’ experience with producing aflatoxin-free
corn on the farm, and more specifically in a given field. Cropping history, soil
type, availability of irrigation, and experience with site-specific production problems will be determining factors (Smith, 1990).
Band application of a nematicide, insecticide, or both at planting is an effective way to protect young plants from early stress and provide a healthy start.
This practice is especially important if nematodes or cutworm problems are a part
of the field history. Additionally, these treatments are very important for minimum tillage where opportunity for carry-over of problems from the previous
year’s crop debris is possible. Many growers also apply pop-up fertilizer at
planting to give the corn plant a fast start and improve vigor of the young
Early control of specific weeds can be achieved by choosing species or weedclass-specific herbicides, preplant incorporated into the top 5-10 cm of soil.
Weed problems are dependent on the crop rotation being practiced, weed species
N. W. WIDSTROM
distribution in the field, effectiveness of the chosen herbicide in controlling weed
species that are present, and grower tillage practices. Weed control is a routine
recommendation to aid in reducing infection by A . Javus (N. C. Aflatoxin
Committee, 1977). In general, weed problems need to be extensive for sufficient
stress on the plant to predispose it to contamination by aflatoxin during kernel
development (Glover and Krenzer, 1980; Bilgrami et al., 1992). Other sporadic
problems can severely affect the level of stress on the young corn plant, such as
thrips and nutrient deficiencies due to heavy rains that leach nutrients from the
soil. These uncertain occurrences do not warrant prophylactic measures because
of economic restrictions.
b. Control during the Growing Season
It has been a commonly held belief that contamination of the corn crop by
aflatoxin is inevitably beyond the control of the producer when conditions conducive to its formation are present. We now know that the risk of A . flavus infection
and aflatoxin contamination can be reduced substantially through a good production management system. Events which place young corn plants under stress can
have a lasting influence on their susceptibility to attack later in the season.
Management toward a healthy crop must begin early. Some stress-inducing
events have no remedy and it is already too late for corrective action when they
occur (Aldrich et al., 1975). Natural events such as flood or hail are typical
examples. Some problems can be diagnosed and corrective action can be taken.
Those for which remedial action may be effective require close scrutiny of the
crop and immediate action. Insects which attack the very young plant and destroy
it completely obviously do not contribute to the aflatoxin problem. Other insects,
such as thrips, attack in cool, dry weather and stunt growth of the young plant.
An application of irrigation water can often break the infestation cycle and allow
plants to recover fully.
The most critical time for the growing plant, from the standpoint of aflatoxin
contamination, is the grain-filling period (Fig. 1). Any insect attack that produces
stress in the plant will increase the risk of Contamination. The best defense
against insects in field corn is host plant resistance, a prophylactic tool that is
seldom supplemented by insecticides. Insecticides can be effective against leaffeeders and cell-sap feeders, but this method is not often used because of the
cost/benefit factor. The greatest impact is made by ear-feeders that feed on
kernels and expose damaged tissue for invasion by even the least aggressive
fungi, such as the Aspergilli. The concurrence of insect damage and fungal
infection of the ear has been recognized for many years (Taubenhaus, 1920). The
concurrence concept was extended to harvested corn (Fennel1 et af.,1975; Shotwell er a l . , 1977) and later to preharvest corn (Anderson et al., 1976; Wilson et
al., 1981a; McMillian et al., 1985b). Even when fields are checked twice weekly
as recommended by extension agronomists (Smith, 1990) there is often little that
THE AFLATOXIN PROBLEM WITH CORN GRAIN
can be done to eliminate the infestation after it is established inside the ear.
Reducing plant stress by irrigation and timely harvest are other measures that
help to minimize aflatoxin contamination.
Diseases, like insects, impose stresses on the growing corn plant that render it
susceptible to attack by a variety of pests and maladies, including infection by
Aspergillus and subsequent aflatoxin contamination. Most disease problems
which plague the corn plant after emergence are not easily remedied. Prophylactic measures may be the most effective in controlling disease, especially the use
of hybrids that are tolerant or resistant.
Those diseases that infect the ear are the most serious contributors to the
aflatoxin contamination problem. A complex of ear pathogens and ear-feeding
insects all interact (Taubenhaus, 1920) to make effective control difficult. Specific organisms such as Helmitithosporium maydis have been associated as predisposing agents with aflatoxin contamination of the ear (Doupnik, 1972). Certain investigators have chosen to deal with the Aspergilli as ear-rot organisms and
evaluate them accordingly (Campbell et al., 1993; Campbell and White, 1994).
The Aspergilli are now classified as causative organisms for both ear rots and
storage rots by authorities (Shurtleff, 1980), but they were considered by mycologists to be weak or nonaggressive pathogens for many years (Payne, 1987).
When diseases do appear, they often occur in localized areas within fields, or
only in certain fields (Smith, 1990).Optimum practices in irrigation and fertilization to assure a nonstressed, healthy growing plant can help to minimize the
spread of existing diseases and may even limit opportunities for others to get an
infection foothold. In essence, management to optimize production is also management to minimize the risk of aflatoxin contamination. Usually by the time
disease symptoms are expressed, it is already too late to reverse the process, and
emphasis should rather be placed on containment or limiting spread and severity.
Any evidence of mold in the ears, found during regular inspections after denting
has begun, should initiate grain sampling and testing for the presence of aflatoxin
Leaf-feeding by lepidopterous insects during all pretassel stages can impose
considerable stress on the plant if damage is extensive. If the infestation is
discovered early and is very heavy, an application of insecticide can be effective
and economical when one considers potential losses. Decisions regarding insecticide applications on a growing corn crop are very difficult and must be carefully
weighed to determine cost benefit, but they must be made quickly, before extensive damage, to be effective. Spot treatment may be effective if infestations are
detected early (Aldrich et al., 1975).
Experiments designed to determine if aflatoxin contamination could be eliminated if insects were removed from the picture by insecticides were conducted by
Lillehoj et a/. ( 1976~)and Widstrom el al. (1976). In both experiments the
insecticide treatments did not completely eliminate insect damage nor did they
N. W. WIDSTROM
preclude A. flavus infection or aflatoxin contamination. Studies by Draughon et
a!. (1983) indicated that certain insecticides were capable of inhibiting aflatoxin
production by A. parasiticus in the laboratory, and to some extent in the field, but
not sufficiently to assure safe use of corn that had been exposed to adequate
inoculum. The application of insecticides to control A. flauus infection would
certainly not be cost effective unless they could be relied upon to eliminate
aflatoxin contamination. Recent studies have confirmed the inadequacy of insecticides as a means to eliminate contamination (Smith and Riley, 1992).
Insects such as Heliothis virescens, an insect closely related to some of the earfeeders in corn, are susceptible to aflatoxin (Gudauskas et al., 1967). Aflatoxin
also is toxic to several other insects (Matsumura and Knight, 1967), suggesting
that A. fluvus or its toxins may function as natural control agents for some insects
(Roberts and Yendol, 197 1). McMillian ef al. (1980~)examined this possibility
for three of corn’s ear feeders and found that dosages sufficient to adversely affect
corn earworm, fall armyworm, and European corn borer ( ~ 2 5 ng
0 g-l) were
much higher than allowed as a contaminant of corn as a feed grain (20 ng g-I).
The toxicity occurs at such a high concentration that it may be of little practical
Maize weevils have greater tolerance to aflatoxin than the lepidopterous insects and can survive on grain with contamination levels exceeding 1 Fg g-1
(McMillian et al., 1981). Corn earworm and fall armyworm have lower tolerances to aflatoxin but, as with the maize weevil, they are more drastically
affected by A. parasiticus isolates and their toxins than those of A. flavus (Wilson
et ul., 1984). Iowa investigators determined that the tolerance of European corn
borers increased with each successive instar, and concluded that levels of toxin
generated under field conditions might occasionally be great enough to adversely
affect the insect, but that the overall influence on the insect population would be
minimal (Jarvis et al., 1984).
As with diseases, the ultimate insect control method is host plant resistance.
The best sources of resistance to various insects will probably be the best option
for control, in that neither plant resistance nor insecticides have eliminated
damage and that plant resistance is the more cost efficient and environmentally
sound. Some resistant germplasm is available for most ear-feeding insects of
importance to corn (Guthrie et al., 1970; Guthrie and Dicke, 1972; Scott and
Davis, 1981; McMillian et al., 1982b; Widstrom et al., 1983, 1992).
Close periodic observations of the corn crop during the early stages of growth
and again during the grain-fill period may be critical to minimizing the risk of
THE AFLATOXIN PROBLEM WITH CORN GRAIN
eventual aflatoxin contamination. Anything that induces plant stress (moisture
deficit, insect infestations, disease incidence, or nutritional deficiency) must be
remedied as soon as possible, to prevent the need for lengthy recovery, which
provides a wider window of opportunity for vulnerability to attack by Aspergilli.
Good management is one of the most important components of producing an
aflatoxin-free corn crop, and at worst, a crop with limited contamination in the
most stressful environments. With respect to plant stress, those practices that
maintain the healthiest highly productive plants also minimize aflatoxin contamination.
The growers who maintain good records on crops that were grown in each of
their fields and on problems that were encountered during the cropping history of
those fields are better able to anticipate problems and take steps to avoid them
when corn is again planted in the rotation. A typical example of such records
would be a field map showing those areas that are droughty and have produced
corn with high levels of aflatoxin in previous years. When droughty areas cannot
be avoided, more intensive monitoring of them may serve as an early warning
system to determine when conditions are favorable for aflatoxin development.
Once these areas are identified, they may be either avoided or eliminated from
the harvest when aflatoxin has been detected during years of marginal contamination.
V. HANDLING THE GRAIN CROP AT HARVEST
Fortunately, rules for harvest management change very little, whether or not
considerations are made for control of aflatoxin Contamination. The basic tenet is
to harvest the crop as soon as possible after physiological maturity to maintain
grain quality and minimize other losses. The major expense variable at harvest is
the consideration concerning artificial drying. This consideration is often a function of weather, especially temperature and moisture, and ultimately the most
critical decision to be made for control of aflatoxin once the crop reaches maturity.
Early or prompt harvest at maturity is critical in obtaining a crop with minimal
aflatoxin Contamination. Delaying the harvest of a crop which is known to have
some contamination can only result in higher amounts of aflatoxin in the harvested grain (Jones e? d.,
1981). Since contamination is cumulative, delay can
only exacerbate the problem on infected ears, even when some resistance to