MYCOTOXIN/SOIL CONTAMINATION

There are two potential effects of mycoherbicides in soils that should be considered:

• the effect of the fungus; its concentration, and 
• its potential for mutation and nonspecificity.  

For a discussion of mutation and nonspecificity, please go to the mycology section of this website.  

On this page we will discuss the known effects of Fusarium mycotoxins on soil, even after the fungus is no longer active. Fusarium species produce a variety of mycotoxins, depending on the enzyme system present in the particular strain of the fungus, the nutrients available, and other factors not clearly understood at this point.  These mycotoxins -which in the case of plant hosts are called phytotoxins- are produced in such quantities that will allow entry into the "host" plant by dissolving cell walls, and assisting the dissolution of cellular material which the fungus then ingests and then replaces, cell by cell, until the host organism dies.

What then, happens to the mycotoxins after the host plant dies? Clearly some of these mycotoxins degrade.  But some would also be left in the soil, or in the detritus of the dead plant, effectively poisoning the soils around it. Some of these mycotoxins are more water-soluble than others, and these would be leached away by rains, contaminating water supplies until they degraded.  Still other mycotoxins are very nonpolar - they won't dissolve easily in water, and would therefore not be leached away.  These latter mycotoxins especially, would constitute a danger to the continued productivity of the soil.

All of this becomes particularly relevant when considering the potential aftereffects of the massive application of Fusarium as a mycoherbicide against drug plants, as has been recently envisioned by by certain US Congresspeople, Dr. David Sands and his "private" company Ag/Bio Con, the United Nations Drug Programme and to a lessening degree by US government entities.  According to the research below, and the testimony of countless farmers after the Peruvian Fusarium epidemic (which they called 'seca-seca'), yields of legitimate crops in the soils previously infected by Fusarium are abysmal, if anything grows at all.  Indeed, as one Peruvian farmer said to us, "the soils act as if they had been poisoned."

Indeed they may have been.

Here, we will defer to the work of the Russian biologist N.A.Krasil'nikov, whose book  Soil Microorganisms and Higher Plants devotes a chapter to the aftereffects of fungal mycotoxins in soils. In 1961, this work was translated by the Israel Program for Scientific Translations for the US National Science Foundation and the USDA. 

N.A.Krasil'nikov classifies organisms such as the Fusarium species as "microbial inhibitors" of plant growth: "Investigations show that microbial inhibitors may poison plants with their toxins under conditions of their natural growth in soil, if favorable conditions for such growth are formed. They suppress germination of seeds, the growth of sprouts and plant growth in general and decrease the total crop. Consequently, when there is a massive growth of these organisms, they may become an important factor in determining the fertility of soil and the crop yield of plants."

In other words, the presence of  active quantities of mycotoxins in the soil, by itself would constitute a danger for successive crops.  Combine this with the presence of the reproducing and mutating fungus (which one would suspect would be immune to the presence of its own mycotoxins), and the problem would be seriously compounded.

The effects described by Krasil'nikov are certainly identical to the effects of Fusarium on coca and other plants in the Peruvian Huallaga Valley: "Certain toxins of microbial origin cause the phenomenon of chlorosis in grapevines, According to our observations, this phenomenon may be due to fungi of the genus Fusarium. We found certain strains, the toxins of which caused the etiolation of shoots, of cuttings, and grape stock, when treated before planting in the soil. The plants that grew from them had light green leaves with a yellowish hue, their development was slow, and other deviations were observed which are characteristic of chlorosis of grape vines (Krasil'nikov and Kublitskaya, 1956)...

This picture of the etiolation of cuttings was observed by us after the treatment of the vine with antibiotics of actinomycete origin. Certain strains of gray and pigmented actinomycetes synthesized substances which inhibit the formation of chlorophyll in the leaves of grapevines. Cuttings, when immersed with their basal ends in the crude fluid culture and subsequently planted in the soil developed and showed obvious signs of etiolation...

Producers of toxic substances are known among various groups of soil microflora. An important place is occupied by representatives of the genus Fusarium. The substances formed by them were obtained in a chemically pure form having a known structure; for example, lateritin, C₆H₄₆O₇N₂ ; avenacein, C2₅H₄₄O₇N₂; fructigenin, C₂₆H₄₄O₇N₂; sambucynin, C₂₄H₄₂O₇N₂, and enniatins, lycomarasmin, yavanicin, etc.

These substances act differently on plants and animals. Some of them are specific (Goiman, 1954)...

Fusaria are very widespread in nature. The probably play an important role in the toxicoses of soils. Their inhibitory effect on the growth of plants was observed by many authors (Rehm, 1953; Laundoldt, 1952; Sukhorukov, 1952). The significance of these fungi for the fertility of soils is not only determined by their ability to synthesize toxins and excrete them into the soil but also by their phytopathogenic properties.

Bilai (1955) described in his monograph many strains of the genus Fusarium which have a deleterious effect on the germination of seeds and on the growth of seedlings of rye, oats, and barley. The products of their metabolism, obtained in the form of filtrates, were tested under various conditions. The results of the author's experiments are given in Table 98.

Table 98
Effect of filtrates of Fusarium cultures on the germination of plant seeds
(in length of plant parts in cm)

As can be seen from the table, the filtrates of some strains affect the seedlings of rye, while others act predominantly on the growth of barley. Certain strains suppress the growth of rye and wheat to the same extent as that of barley or oats.

Klechetov (1926) in studying the phenomenon of the flax exhaustion of soils found the growth of the fungi Fusarium, Thielaviopsis basicola, Cladosporium herbarum, Alternaria, and Macrosporium in these soils; these fungi, according to the author, form toxic substances and are the reason for the death of the sown flax.

A considerable role in the exhaustion of soils and in the lowering of plant yields is attributed in the literature to the fungi of the genus Fusarium. Kvashina (1938), Kurtesova (1940), and Ioffe (1950).

Kublitskaya (1955) studied the degree of the distribution of fungi of the genus Fusarium in the soils of Central Asia (Uzbek SSR) under grapes. She isolated 52 cultures and many of them proved to be toxic for grapevines, causing poisoning and death to the cuttings and stock under the conditions of growth in soil. Certain strains caused chlorosis under experimental conditions." Soil Microorganisms and Higher Plants N. A. Krasil'nikov, Academy of Sciences of the USSR, Institute of Microbiology Published by the Academy of Sciences of the USSR, Moscow 1958, published for THE NATIONAL SCIENCE FOUNDATION, WASHINGTON, D.C. and the USDA, USA by THE ISRAEL PROGRAM FOR SCIENTIFIC TRANSLATIONS.1961.

[N.A.Krasil'nikov also provides us with some disturbing, but relevant information about the danger of fertilizing such toxic soils with chemical fertilizers:

As can be seen from the data given, the greatest number of inhibitors was found in soils cultivated to a limited extent. Mineral fertilizers do not diminish but, on the contrary, they noticeably increase the content of inhibitors."

This may explain why some Peruvian farmers believe that the fungus was marketed to them in adulterated fertilizers, although even some Peruvian shopkeepers have alleged that the fungus was spread in this fashion.

"It was experimentally established that microbial inhibitors form toxic substances directly in the soil in which they grow.

If these organisms are introduced into nontoxic or inactivated soil and the soil is incubated under certain conditions of humidity and temperature, then after a certain time it will become toxic for these or other plants or for certain species of microorganisms, depending on the peculiarities of the inhibitor.

Rybalkina (1938 a) observed the appearance of toxicosis in flax-exhausted soil upon growth of the fungus (Fusarium lini).

Mirchink (1956) incubated soil (podsol) with fungi-inhibitors and she observed the appearance of toxicosis. In soils in which the fungus Penicillium cyclopium grew abundantly if artificially introduced, seeds of wheat did not germinate at all or germinated in small numbers (Figure 91). Other species of fungi isolated from podsol soils also poisoned the soil but to a lesser degree. On such soils germinating wheat seedlings constituted 15-60% of the number of seedlings in normal control soil."  Soil Microorganisms and Higher Plants N. A. Krasil'nikov, Academy of Sciences of the USSR, Institute of Microbiology Published by the Academy of Sciences of the USSR, Moscow 1958, published for THE NATIONAL SCIENCE FOUNDATION, WASHINGTON, D.C. and the USDA, USA by THE ISRAEL PROGRAM FOR SCIENTIFIC TRANSLATIONS.1961.]