The dynamics of the blister rust life cycle reveal the major climatic limitations for the spread of blister rust to whitebark pine. In pines, the blister rust cycle (Fig. 8.3) commences in the summer a year or more after pine infection when spermatia, the initial spore stage, form in nectarlike droplets that ooze from the bark. These spores are transmitted by rain or insects that are attracted to the nectarlike exudate and travel from canker to canker. If successfully sperma-tized, the aeciospores push through the bark of the tree the following spring to form cankers with the onset of warmer weather (Van Arsdel et al. 1956a, Hunt 1983). Saturated atmospheric conditions persisting for 5 or more hours are required for these spores to mature and disseminate, conditions that commonly occur in the GYE during the spring and summer following the initiation of snowmelt when moisture from evaporating snow is freely available (P. Farnes, pers. comm. 1998). These spores, which are capable of long-distance travel, infect Ribes spp. through the leaf stomates, leading to the production of uredinia on the undersides of leaves. Because of the frequency of occurrence of these climatic conditions, and the durability of aeciospores, it is unlikely that aeciospore production is limiting in the GYE.
Rust damage to Ribes leaves varies considerably among species and even individuals, with some Ribes leaves exhibiting early necrosis and others revealing little or no effect from the infection (Mielke 1943, Kinloch and Dulitz 1990). Significantly, leaves that die early in the season are unavailable to support the later spore stages in the blister rust cycle that infect pines. In the event of favorable climatic conditions, several generations of urediniospores can be produced on the same leaves over the course of a single summer, and it is the intensification of this stage that in part controls the degree of pine infection if the rust cycle is completed. Abundant uredinial and telial multiplication is more a function of the number of repeated moist periods distributed over the summer (thereby allowing several generations of urediniospores to be formed) than net seasonal moisture (Mielke et al. 1937).
Urediniospore formation requires from 7 to 24 days and the rapidity of formation is temperature regulated. For development to occur, daytime temperature needs to fall between 12 and 28 °C, with maximum development of fertile spores occurring at 20 °C,
and higher temperatures generally leading to faster development (Lachmund 1934, Van Arsdel 1965). These spores multiply and some spread to nearby Ribes leaves and bushes during periods of near atmospheric saturation or in the presence of free water on the leaves, which occurs many nights in valley troughs. Urediniospore conversion to the telial structure requires from 4 to 12 hours of saturated conditions (Van Arsdel 1965).
The formation of teliospores is likewise governed by temperature and moisture. Cooler temperatures result in maximum spore production and fertility, such as those commonly found in the GYE in late summer and early autumn. At least 2 weeks are required between the time of urediniospore conversion to telia and teliospore formation (Van Arsdel et al. 1956a). Three consecutive days where temperatures are in excess of 28 °C during the period of development are sufficient to prevent telial formation. This factor explains why blister rust is more commonly found at sites with low daily maximum temperatures, or with negative radiation budgets, common to small forest gaps (Van Arsdel 1961). Sites at high latitudes and elevations, including many within the whitebark pine zone in the GYE, fit these criteria.
The final spore stage involves the production of basidia from mature teliospore columns, which dangle from beneath Ribes leaves. In the same period, basidia must also germinate and release basidiospores, which infect pines and complete the blister rust cycle (Kimmey and Wagener 1961). Following germination of the sporid-ium on the surface of the needle, a germ tube is produced and enters the pine through the needle stomates. Inside the needle, a mass of hyphae is produced that grows down the length of the needle and enters the bark through the needle fascicle (Mielke 1943). From teliospore germination to pine infection, free water or relative humidity in excess of 97% must be present for a duration of 24 to 48 hours, and temperatures must fall below 20°C. However, longer
Table 8.1. Spore Stage Development: Time, Temperature, and Humidity Requirements.
Time Required for Formation
Temperature Range for Spore Formation
Required Precipitation Conditions for Spore Formation
5 hours 4-12 hours 24-48 hours
Humidity>97% or free water on Ribes leaves Humidity>97% or free water on Ribes leaves
Humidity>97% or free water on Ribes leaves and pine needles periods of available moisture lead to greater pine infection. Because the temporal requirements for formation of the final spore stage and pine infection are greatest, this phase of the blister rust cycle is probably most limiting in the GYE.
Where prolonged periods of rain are most likely to lead to spore germination and pine infection (Spaulding and Rathbun-Gravatt 1926, Hirt 1942, Mielke 1943), heavy rains may prevent sporidia transport or may wash away sporidia before the mycelium has had a chance to attach to the pine needle. The timing of the critical moist period is also important in relation to the diurnal cycle. Van Arsdel et al. (1956) found that the majority of extremely fragile blister rust sporidia are released at night and thus are reliant on gentle nighttime air currents for their spread. Sporidia viability has also been shown to be damaged by direct sunlight (Van Arsdel 1967).
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