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• root rots(e.g. P. cinnamomi, a major pathogen of the
  root tips of avocado trees, and of Eucalyptusvegeta-
  tion in Australia);


• cankers, which develop near the base of trees (e.g.
  P. ramorumwhich is currently devastating the native
  oak woodlands of the coastal mist belt of California
  and southern Oregon, causing sudden oak death);


• collar rotsand crown rots, which develop at the base
  of the stems of woody and herbaceous plants (e.g.
  P. capsici, which attacks tomatoes, cucumber, water-
  melon, squash, and pumpkins);


• aerial blights(e.g. P. nicotianaeon poinsettias and
  many other plants);


• fruit rotsof many plants (e.g. P. palmivora on
  Capsicumpeppers, papaya, citrus, etc.).

Apart from the economic damage caused by these

organisms, Phytophthora spp. are notable because

they are not fungi at all. Instead, as we have noted

repeatedly, they are members of the Oomycota, within

the Kingdom Straminipila (which includes the orange-

colored algae) and they have several distinctive features,

such as cellulosic cell walls, diploid vegetative stages,

motile flagellate spores, and biochemical features that

make them insensitive to many conventional fungicides.

Here we will consider two species of Phytophthorathat

cause very different types of disease – P. infestansand

P. ramorum. However, they also have several features

in common, including the fact that they are spreading

globally and causing serious damage to wild plants and

crop plants. They demonstrate the continuing threat

posed by members of this genus.

Phytophthora infestans

The centre of origin of wild potatoes is central Mexico,

and this is almost certainly the centre of origin of P.

infestans, because this fungus is heterothallic, requiring

two mating types for sexual reproduction (mating

types A1 and A2) and both of these are found com-

monly in central Mexico. The fungus is known to have

reached Europe, including Britain, before the 1840s

when it caused devastating losses of potato crops. But

until the early 1980s only the A1 mating type was found

commonly outside of Mexico. Then the A2 mating type

FUNGI AS PLANT PATHOGENS 299

61.9%

61.2%

76.4%

56.3%

Northeast

Western

Central

Southern

Shaded states =

participating states

Fig. 14.18Percentage of pastures testing positive for Neotyphodiumendophytes in a survey conducted by United States
Department of Agriculture in 1998. (Source: http://www.aphis.usda.gov/vs/ceah/Equine/eq98endoph.htm))