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98 CHAPTER 5

the spores and were surrounded by flagellarmembranes.

This difference in origin of the flagellar membrane and

the plasma membrane that surrounds the rest of the

zoospore body could be significant for zoospore func-

tion, because there is evidence that different putative

chemoreceptors occur on the flagella compared with

on the body of a zoospore (Chapter 10).

The cleavage and release of zoospores of the

Oomycota is strongly influenced by environmental

factors; typically, the sporangia must be washed

to remove nutrients and then flooded with water to

induce the cleavage and release of zoospores. Most

members of the Oomycota have remained essentially

aquatic, producing zoospores from sporangia that

remain attached to the hyphae; but some of the

plant-pathogenic Phytophthoraspecies and the related

downy mildew fungi have detachable, wind-dispersed

sporangia. In the case of Phytophthora infestans (potato

blight; Fig. 5.17) and P. erythroseptica(pink rot of

potatoes) these wind-dispersed sporangia will undergo

cleavage and release their zoospores when incubated

in water at a temperature of 12° or lower, but at higher

temperatures (around 20°) the detached sporangia

exhibit “direct” germination by producing a hypha.

This temperature-regulated development seems to

be functionally significant. In Britain and other cool

regions, zoosporesare thought to be the main infect-

ive agents of P. infestansbecause the sporangia are pro-

duced, dispersed and land on potato leaves early in the

growing season when the cool, wet conditions would

favor zoospore production. But “direct” germination

of sporangia (by hyphal outgrowth) might be more

important for infection of the tubers later in the

growing season when the temperatures are warmer.

Some of the downy mildew pathogens such as

Pseudoperonospora humulion hops and Plasmopara

viticolaon grapevine typically produce zoospores for

infection through the host stomata. However, some

other downy mildew pathogens (Bremia lactucaeon let-

tuce, Peronospora parasiticaon cruciferous hosts) have

sporangia that usually or always germinate by hyphae,

which then invade through the host epidermal walls.

Several Oomycota are sensitive to the antibacterial

agent streptomycin. At high concentrations this

compound is toxic, but at sublethal concentrations

it interferes with zoospore cleavage, causing the spo-

rangium contents to be released as a multinucleate

mass with several flagella and incapable of coordin-

ated swimming. For this reason, streptomycin was used

at one time to control Pseudoperonospora humuli(hop

downy mildew), although now it has been replaced

by conventional fungicides (Chapter 17). The mode

of action of streptomycin on sporangia is unclear,

but it does not necessarily involve the inhibition of

Fig. 5.16(a) Sporangia of Mucorcontaining darkly pigmented spores. (b) Mid-cleavage stage in the sporangium of
Gilbertella persicaria(Zygomycota). Cleavage vesicles (CV) fuse and extend between the nuclei (N) to separate the
sporangium contents into uninucleate spores. ER =endoplasmic reticulum; L =lipid; M =mitochondrion. (Courtesy of
C.E. Bracker; from Bracker 1968.)

(a) (b)