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Pathogen Biology

There are two closely related fungi that cause rust on soybean: Phakopsora pachyrhizi, sometimes referred to as the Asian or Australasian soybean rust pathogen, but which now also occurs in the western hemisphere, and P. meibomiae, the so-called New World soybean rust pathogen, which is found only in the western hemisphere. Except for a few minor characteristics, the two fungi appear morphologically identical, but P. pachyrhizi is much more aggressive on soybean than P. meibomiae. To date, P. meibomiae has not been documented to cause significant yield losses in Central and South America. The two species can be distinguished by using DNA analysis protocols. Like other rusts, the soybean rust pathogens are obligate parasites that require a living host to grow and reproduce. They can survive away from the host as urediniospores for only a few days under natural conditions.

Both soybean rust pathogens, to the best of our knowledge, produce only two types of spores: urediniospores and teliospores (Figure 15). This contrasts with other rusts, which can have up to five spores stages (for example, wheat stem rust). For soybean rust, like most rusts, the uredinial stage is the repeating stage. This means that urediniospores can infect the same host on which they were produced (soybean) during the same season. Epidemics can develop quickly from only a few pustules because spore-producing pustules are produced in as little as 7 to 10 days after infection, and each pustule can produce hundreds of urediniospores.

Fig. 15

Teliospores are produced in old lesions, but they do not appear to germinate in nature, and no alternate host, nor aecia or spermogonia are known. Without germination of teliospores, sexual reproduction cannot take place. Lack of sexual reproduction should limit variability of the rust fungus, but nevertheless there is substantial variability in P. pachyrhizi with respect to virulence. This has limited the use of single genes for resistance in soybean, because in a short time new isolates of the pathogen arise that overcome the resistance gene. It is not known how this variability originates in P. pachyrhizi. Wheat stripe rust, Puccinia striiformis, has a similar life cycle as P. pachyrhizi with no functioning telial stage and therefore no sexual reproduction, but has many races. It may be that each resistance gene is so specific that a single mutation in the right gene of the fungus allows it to be virulent on hosts with the new resistance gene.

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