POTATO LATE BLIGHT - The Epidemiology of Plant Diseases - page 448

(a) Survival of asexual structures

Overwintering of free-living sporangia and mycelium is limited. Although sporangia
can survive for days or weeks in moist soil (Andrivon, 1995) they do not survive for
long periods and especially they do not survive drying conditions (Fernández-Pavía
et al ., 2004). However, mycelium in infected viable tubers survives well from one
season to the next, whether the tubers are in temperature controlled storage, in a pit
(clamp), or in soil that does not freeze (Zwankhuizen et al ., 1998; Kirk et al ., 2001).
From November through April, temperatures at the base and at the centre of
discarded potato cull piles that varied from 1 to 15 tons rarely dropped below 0°C
(Kirk, 2003). Infected tubers that survive from one season to the next can produce
plants that become infected (Van Der Zaag, 1956). However, the efficiency of
transmission is low - most infected tubers do not initiate epidemics.
Survival of detached P . infestans sporangia in the atmosphere is limited
(Mizubuti et al ., 2000; Sunseri et al ., 2002). For airborne sporangia, temperature,
relative humidity and solar radiation affect survival. Since the work of Crosier
(Crosier, 1934), plant pathologists have questioned the ability of sporangia to
survive in a dry atmosphere. While some workers concluded that sporangia could
survive for several hours (De Weille, 1963; Rotem and Cohen, 1974) others felt that
sporangia could survive only minutes (Crosier, 1934; Glendinning et al ., 1963;
Warren and Colhoun, 1975). It turned out that a factor leading to different results
and different conclusions by different groups of workers was the rehydration rate of
dried sporangia. Dried sporangia that were rehydrated rapidly died, whereas dried
sporangia that rehydrated slowly were more likely to survive (Minogue and Fry,
1981). Nonetheless, sporangia survived longer at high relative humidities than at
low humidities and sporangia survived better at 15 to 20 C. Half-lives

of dried sporangia ranged from 2 to 6 hours at temperatures ranging from 15 to 30

° C than at 30 °

° C

and at relative humidities from 40 to 88% (Minogue and Fry, 1981).
Solar radiation is one of the most important environmental factors that influences
sporangia viability. Exposures of field-produced sporangia to direct solar radiation
during sunny days caused viability to drop from approximately 70% to 0.3% within
one hour. During overcast conditions, however, viability remained practically
constant for a period of at least three hours (Mizubuti et al ., 2000). Even in surface
water normally formed in irrigated fields, solar radiation was detrimental to
sporangia (Porter and Johnson, 2004). Survival of detached sporangia under field
conditions, varied from 0 to 20 days depending on the amount of solar irradiance
(Porter and Johnson, 2004).
Sporangia survive much longer in moist soil than in the atmosphere (Andrivon,
1995). Whereas survival of sporangia in the atmosphere is measured in hours,
survival in soil is measured in days and weeks with longer survival in moist than in
dry soils. Often, survival of sporangia is determined by 'infectivity' (determining
how long a particular soil contains a propagule that can infect a potato tuber or
potato leaf). In soil sporangia can survive for approximately 40 days and zoospores
for up to 10 days (Zan, 1962). Factors associated with reduced infectivity include
low pH and aluminum toxicity as well as moisture (Andrivon, 1995). Additionally,
associated soil microbes lessen survival of sporangia in soil.