**2. Contamination**

The Asian soybean rust is one of the most destructive diseases of soybean because it produces a high amount of airborne spores that can infect large areas of soybeans and cause significant yield loss. The fungal spores (uredospores) are deposited on leaves in the lower region of the canopy through the rain or wind transport from nearby plants during the growing season (Huber & Gillespie, 1992). The figure 1 shows a soybean leave contaminated with Asian soybean rust.

The *P. pachyrhizi* is highly moisture dependent, requiring at least 6 hours of free moisture on the lower trifoliolates to starts the contamination. Warm temperature is ideal, but not limiting, since the disease can be established between 15 °C and 30 °C (Embrapa, 2005). These moist conditions can be achieved through any form of wetness as drizzle, mist, fog or dew and the minimum duration of wetness is dependent on the ambient temperature after spore deposition (Schmitz & Grant, 2009).

Fig. 1. (Godoy et al. 2009). Soybean leaf contaminated with Asian soybean rust.

Early rust symptoms are characterized by small dots of 1 to 2 mm of diameter, darker than the healthy leaf tissue with a greenish to greenish gray coloration. On the local corresponding at the dark spot, there is initially a tiny lump, like a bubble formed by burning, showing the early

Soybean plants are susceptible to the fungus at all growth stages. As a general rule, the earlier a crop is attacked, the higher will be the loss (Mendes et al., 2009), however, if the attack occurs at flowering and pod filling stage, which is commonly observed in soybean

The Asian soybean rust is one of the most destructive diseases of soybean because it produces a high amount of airborne spores that can infect large areas of soybeans and cause significant yield loss. The fungal spores (uredospores) are deposited on leaves in the lower region of the canopy through the rain or wind transport from nearby plants during the growing season (Huber & Gillespie, 1992). The figure 1 shows a soybean leave contaminated

The *P. pachyrhizi* is highly moisture dependent, requiring at least 6 hours of free moisture on the lower trifoliolates to starts the contamination. Warm temperature is ideal, but not limiting, since the disease can be established between 15 °C and 30 °C (Embrapa, 2005). These moist conditions can be achieved through any form of wetness as drizzle, mist, fog or dew and the minimum duration of wetness is dependent on the ambient temperature after

Fig. 1. (Godoy et al. 2009). Soybean leaf contaminated with Asian soybean rust.

Early rust symptoms are characterized by small dots of 1 to 2 mm of diameter, darker than the healthy leaf tissue with a greenish to greenish gray coloration. On the local corresponding at the dark spot, there is initially a tiny lump, like a bubble formed by burning, showing the early

fields, the yield reducing can be higher than in others stages (Kawuki et al., 2004).

**2. Contamination** 

with Asian soybean rust.

spore deposition (Schmitz & Grant, 2009).

formation of the fruiting structure of fungi. As soon as the death of infected tissues, the blemishes increase in size and acquire a reddish brown color. The uredospores, initially has a crystalline color, become beige and accumulate around the pores or are carried by the wind and the number of uredias per point can vary from one to six. The uredias that no longer sporulete shows the pustules with open pores, which allows distinguish them from bacterial pustule, which often causes confusion comparison (Bromfield, 1980; Embrapa, 2004).

The infection causes rapid browning and premature leaf fall, preventing the full grain formation. The earlier the defoliation, smaller is the grain size and lower is the yield and quality. In severe cases, when the disease reaches the stage of the soybean pod formation, it can cause abortion and drop of the pods, resulting in a total loss of income (Constamilan, 2002; Godoy & Canteri, 2004; Soares et al., 2004).

The life cycle is typical of the majority of other rust fungi (Fig. 2) and their uredospores are easily transported by air currents and disseminated hundreds of kilometers in few days (Tremblay et al., 2010).

Once germination occurs, the uredospore produces a single germ tube (GT) that grows across the leaf surface until it reaches an appropriate surface where an appressorium (AP) forms. This penetration occurs between 7-12h after the spore lands on the leaf adaxial surface. Appressoria form over anticlinal walls or over the center of epidermal cells, but rarely over stomata, in contrast to the habit of many other rusts. Thus, penetration is direct rather than through natural openings or through wounds in the leaf tissue. Approximately twenty hours after the spore landing, the *penetration hyphae* (PH), stemming from the appressorium cone, pass through the cuticle to emerge in the intercellular space where a septum is formed to produce the *primary infection hypha* (IH). This IH grows between palisade cells to reach the spongy mesophyll cells where it forms the haustorium (H) (Tremblay et al., 2010).

Where: GT, germ tube; AP, appressorium; PH, penetration hyphae; IH, infection hyphae and H, haustorium.

Fig. 2. (Hahn, 2000 apud Tremblay et al., 2010). Internal structure of a typical dicotyledon leaf showing the different cell layers and infection by a rust fungus.

Once this first stage has been reached, about 4 days after spore landing, additional hyphae emerge and spread through the entire spongy mesophyll layer of cells where many other haustoria are formed. At approximately 6 days after infection, some necrosis of epidermal cells occurs which is visible at the adaxial surface of the leaves (Fig. 3a). Hyphae aggregate and a uredinium arise in the spongy mesophyll cell layer. Uredinia can develop 6-8 days after spore landing and development might extend up to 4 weeks (Tremblay et al., 2010).

The first uredospores produced by the uredinium emerge at the abaxial leaf surface in 9-10 days after spore landing and spore production can be observed for up to 3 weeks. High rate of sporulation is typical of a susceptible reaction where lesions on the upper surface of the leaf are tan (Fig. 3b). Plants classified as resistant develop a dark, reddish-brown lesion with few or no spores (Fig. 3c) (Tremblay et al., 2010).

Soybean rust diagnosis is usually performed by experienced plant pathologists or plant disease diagnosticians, but nowadays, several technologies are being performed as crop health sensor, either optical or electronic or bio-electronic based to improve the perform crop disease diagnosis (Cui et al., 2010).

A useful tool that can be used as many by experienced professionals as amateurs is the diagrammatic scale to assess the severity of rust. It is very important once provides data on the severity of contamination in the plantation and the result should be informed when the competent organs are contacted, as well as help to define the goals for fungicides treatment. There are several types of scales such as developed by Godoy et al. (2006) (Fig. 4a) and Martins et al. (2004) (Fig. 4b) witch is highly recommended to be used together.

Once this first stage has been reached, about 4 days after spore landing, additional hyphae emerge and spread through the entire spongy mesophyll layer of cells where many other haustoria are formed. At approximately 6 days after infection, some necrosis of epidermal cells occurs which is visible at the adaxial surface of the leaves (Fig. 3a). Hyphae aggregate and a uredinium arise in the spongy mesophyll cell layer. Uredinia can develop 6-8 days after spore landing and development might extend up to 4 weeks (Tremblay et

The first uredospores produced by the uredinium emerge at the abaxial leaf surface in 9-10 days after spore landing and spore production can be observed for up to 3 weeks. High rate of sporulation is typical of a susceptible reaction where lesions on the upper surface of the leaf are tan (Fig. 3b). Plants classified as resistant develop a dark, reddish-brown lesion with

Soybean rust diagnosis is usually performed by experienced plant pathologists or plant disease diagnosticians, but nowadays, several technologies are being performed as crop health sensor, either optical or electronic or bio-electronic based to improve the perform

A useful tool that can be used as many by experienced professionals as amateurs is the diagrammatic scale to assess the severity of rust. It is very important once provides data on the severity of contamination in the plantation and the result should be informed when the competent organs are contacted, as well as help to define the goals for fungicides treatment. There are several types of scales such as developed by Godoy et al. (2006) (Fig. 4a) and

(3a)

Martins et al. (2004) (Fig. 4b) witch is highly recommended to be used together.

al., 2010).

few or no spores (Fig. 3c) (Tremblay et al., 2010).

crop disease diagnosis (Cui et al., 2010).

(3b)

(3c)

Fig. 3. (Tremblay et al., 2010). Symptoms observed on soybean leaves. (a) Yellow mosaic discoloration (b) Tan lesions and (c) reddish-brown lesions.

(4a)

2.4% 15.2% 25.9% 40.5% 66.6%

Fig. 4. Two diagrammatic scales of Asian soybean rust severity with percentage that represents the area of disease contamination.
