Puccinia tritici Biology
The pathogen Puccinia triticina needs same environmental conditions than the wheat leaf. The fungus is able to infect within dew periods of three hours or less at temperatures of about 20°C the plant tissue; however, more infections occur with longer dew periods. At cooler temperatures, longer dew periods are required, for example, at 10°C a 12-hour dew period is necessary. Few if any infections occur where dew period temperatures are above 32°C (Stubbs et al., 1986) or below 2°C.
Most of the severe epidemics occur when uredinia and/or latent infections survive the winter at some threshold level on the wheat crop, or where spring-sown wheat is the recipient of exogenous inoculum at an early date, usually before heading. Severe epidemics and losses can occur when the flag leaf is infected before anthesis (Chester, 1946). Puccinia triticina is primarily a pathogen of wheat, its immediate ancestors and the man-made crop triticale.
The fungus produces its sexual gametes (pycniospores and receptive hyphae) on the alternate host. Most rust researchers assume that Thalictrum speciosissimum (in the Ranunculaceae family) is the primary alternate host for P. recondita f. sp. tritici in Europe. The alternate host is infected when the teliospores germinate in the presence of free moisture. Basidiospores (1n) are produced that are capable of being carried a short distance (a few metres) to infect the alternate hosts. Approximately seven to ten days following infection, pycnia with pycniospores and receptive hyphae appear. These serve as the gametes, and fertilization occurs when the nectar containing the pycniospores is carried to receptive hyphae of the other mating type by insects, by splashing rain, or by cohesion. The aecial cups appear seven to ten days later on the lower surface of the leaf, producing aeciospores that are windborne and that cause infection by penetrating the stomata of the wheat leaves (Figure 13.1). The distances travelled by aeciospores appear to be relatively short.
Life cycle (Brown rust)
The Figure beside shows the life cycle for P. triticina and P. triticiduri and the disease cycle for wheat leaf rust. The time for each event and frequency of some events (sexual cycle, wheat cropping season and green-bridge) may vary among areas and regions of the world.
The alternate host currently provides little direct inoculum of P. triticina to wheat, but may be a mechanism for genetic exchanges between races and perhaps populations. The pathogen survives the period between wheat crops in many areas on a green-bridge of volunteer (self-sown) wheat (see section "Epidemiology"). Inoculum in the form of urediniospores can be blown by winds from one region to another. The sexual cycle is essential for P. triticiduri. Teliospores can germinate shortly after development, and basidiospore infection can occur throughout the wheat-growing cycle.
Urediniospores initiate germination 30 minutes after contact with free water at temperatures of 15° to 25°C. The germ tube grows along the leaf surface until it reaches a stoma; an appressorium is then formed, followed immediately by the development of a penetration peg and a sub-stomatal vesicle from which primary hyphae develop. A haustorial mother cell develops against the mesophyll cell, and direct penetration occurs. The haustorium is formed inside the living host cell in a compatible host-pathogen interaction. Secondary hyphae develop resulting in additional haustorial mother cells and haustoria. In an incompatible host-pathogen response, haustoria fail to develop or develop at a slower rate. When the host cell dies, the fungus haustorium dies. Depending upon when or how many cells are involved, the host-pathogen interaction will result in a visible resistance response (Rowell, 1981, 1982).
Spore germination to sporulation can occur within a seven- to ten-day period at optimum and constant temperatures. At low temperatures (10° to15°C) or diurnal fluctuations, longer periods are necessary. The fungus may survive as insipid mycelia for a month or more when temperatures are near or below freezing. Maximum sporulation is reached about four days following initial sporulation (at about 20°C). Although the number can vary greatly, about 3 000 spores are produced per uredinium per day. This level of production may continue for three weeks or more if the wheat leaf remains alive that long (Chester, 1946; Stubbs et al., 1986). Uredinia (pustules) are red, oval-shaped and scattered, and they break through the epidermis (Plate 12). Urediniospores are orangered to dark red, echinulate, spherical and usually measure 20 to 28 µm in diameter (Plate 13). The teliospores (Plate 14) are dark brown, two-celled with thick walls and rounded or flattened at the apex (Plate 15). Puccinia triticiduri differs from P. triticina in requiring 10 to 12 days for appearance of urediniospores, and initial teliospore production often occurs within 14 days of the initial infection. The uredinia are yellowish-brown and produce many fewer urediniospores per uredinia, and within a few days the lesion primarily produces teliospores. Also P. triticiduri infections are likely to be on the lower leaf surface.
The teliospores of P. triticina are formed under the epidermis with unfavourable conditions or senescence and remain with the leaves. Leaf tissues can be dispersed or moved by wind, animals or humans to considerable distances. Basidiospores are formed and released under humid conditions, which limit their spread. Basidiospores are also hyaline and sensitive to light, further limiting travel to probably tens of metres. Aeciospores are more similar to urediniospores in their ability to be transported by wind currents, but long-distance transport has not been noted for some reason. Puccinia triticiduri will produce abundant teliospores within weeks of the initial infection, producing a dark ring telia around each infection site.
Source: The wheat rusts: R.P. Singh, J. Huerta-Espino, A.P. Roelfs