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Symptoms and Signs

The first symptoms of soybean rust caused by Phakopsora pachyrhizi begin as very small brown or brick-red spots on leaves (Figure 2). Symptoms caused by P. meibomiae are similar to those of P. pachyrhizi, but this lesson will focus on P. pachyrhizi because most of the research and observations have been made with this species. In the field, these spots usually begin in the lower canopy at or after flowering, although seedlings can be infected under certain circumstances. Often the first lesions appear toward the base of the leaflet near the petiole and leaf veins. This part of the leaflet probably retains dew longer, making conditions more favorable for infection. Lesions remain small (2-5 mm in diameter), but increase in number as the disease progresses. Pustules (Figure 3), called uredinia, form in these lesions, mostly on the lower leaf surface, and they can produce many urediniospores. The raised pustules can be seen with the unaided eye, especially when sporulating (Figure 4). Even though the lesions are small, each lesion often has several pustules (uredinia) (Figure 5). Lesions can be completely covered in urediniospores when the pustules are active (Figure 6). Soybean rust urediniospores are pale yellow-brown to colorless, with an echinulate (short spines) surface ornamentation (Figures 7 and 8). This coloration is different from many other rust pathogens whose spores are often reddish-brown (rust colored). Germination of P. pachyrhizi urediniospores occurs through an equatorial (central) pore, producing a germ tube that ends in an appressorium, which the fungus uses to penetrate the host directly or through a stoma (Figure 9).


Fig. 1	Fig. 2	Fig. 3

Fig. 4	Fig. 5	Fig. 6

Fig. 7	Fig. 8	Fig. 9

As more and more lesions form on a leaflet, the affected area begins to yellow, and eventually the leaflet falls from the plant (Figure 1). While soybean rust usually begins in the lower canopy, it quickly progresses up the plant until all of the leaves have some level of disease. Severely diseased plants may become completely defoliated. The loss of effective leaf tissue results in yield reductions from both fewer and smaller seed. Yield losses as high as 30 to 80% have been reported, but the amount of loss depends on when the disease begins and how rapidly it progresses. Besides leaves, soybean rust can also appear on petioles, stems, and even cotyledons, but most rust lesions occur on leaves.

Lesions may be either tan (Figure 10) or red-brown (Figure 11). Tan lesions have many pustules that produce numerous urediniospores. Red-brown lesions, thought to be a moderate resistance reaction, have only a few pustules that produce only a few urediniospores. As will be discussed in the Disease Management section, this lesion type depends on the strain of the pathogen, and may appear on the same leaf with tan lesions, or tan lesions may turn red-brown with age. Symptoms and signs on other hosts, such as kudzu, are similar, although lesion size may differ.


Fig. 10	Fig. 11	Fig. 12

Fig. 13	Fig. 14

As pustules age, they may turn black (Figure 12). This is caused by the formation of a layer of teliospores in the pustules, turning pustules from uredinia into telia (Figure 13 and 14). Teliospores have two functions: survival of the fungus in the absence of a living host (overseasoning) and sexual reproduction. The thick walls of the teliospores protect the fungus from the environment and attack by other organisms. In rusts, the teliospores germinate forming a basidium and four basidiospores during which sexual recombination occurs. Germination of P. pachyrhizi teliospores has been observed only in the laboratory and does not seem to make a significant contribution to the perpetuation of this disease in the field.

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