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type and transfer the nucleus through a pore in the wall.

This nucleus then divides and migrates through the

monokaryotic hyphae that grew from the basid-

iospores, producing a dikaryotic mycelium. By this

time, the mycelium has grown through the thickness

of the leaf and produces a further sporing structure

that erupts through the lower epidermis – an aecium

containing aeciospores(Fig. 14.23e,g). The dikaryotic

aeciospores can only infect wheat, and they give

rise to infections on which uredospores will develop,

thereby completing the cycle.

A potential solution to breaking the cycle of black stem

rust would be to eradicate barberry bushes so that the

fungus cannot infect its alternate host. A major barberry

eradication program was attempted in North America

in the last century, but was unsuccessful because the

epidemic spreads progressively northwards from the

early-sown crops in Mexico to successively later-sown

crops in the more northerly regions of the USA.

Powdery mildew fungi

The powdery mildew fungi (Ascomycota) cause signi-

ficant epidemics in most seasons if they are not

treated with fungicides, and can be particularly serious

in dry, hot summers. Several crop and wild plants

are infected by these fungi, which in general are host-

specific. Common examples include powdery mildew

of roses (Sphaerotheca pannosa), of gooseberries (S.

mors-uvae), and of hawthorn bushes (Podosphaera clan-

destina). Most people will have seen one or more of these

fungi because they produce white, powdery disease

pustules on the surfaces of many leaves and fruits. But

the most economically damaging species is Blumeria

graminis(previously called Erysiphe graminis), which

causes powdery mildew of cereals (Fig. 14.24). Like

the rust fungi, B. graminisundergoes multiple cycles of

infection in a single season and can cause serious

FUNGI AS PLANT PATHOGENS 305

Fig. 14.23(continued) (d) Lesions containing spermogonia on the upper surface of a barberry leaf. (e) Aecia erupting
through the lower epidermis of a barberry leaf. (f ) A spermogonium, showing the tiny spermatia and receptive hyphae.
(g) Cross-section of an aecium.