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Re: Potato problem

Hi Jane,

> My potatoes started out looking great--they're growing in a 3 foot vertical
> wire cage with a mixture of compost/soil.  But now the leaves have developed
> black spots and are starting to yellow.  I don't see any pests and I've looked
> closely for mites, etc.  Is this a common problem for potatoes or should I
> take an affected cutting into the Extension office for identification?

> *-*-*-*-*-*-*-*-*-*-*
> Jane Patterson
> Baton Rouge, LA
> zone 8b

Sounds like early blight.  It is caused by /Alterniaria solani,/ a fungus.  It
usually starts when the weather gets warm, about when the potatoes begin to
flower. Also effects tomato leaves and fruit about the same time.  First you see
a few brown/black spots on the older leaves, with a concentric "target" pattern.
With time they enlarge and run together, the surrounding tissue yellows and
eventually the leaf drops.  Continues to infect younger leaves until the whole
plant is effected.  Dark, sunken lesions appear on the stems, starting near the
soil level.  Sound like it to you?  By all means take a leaf to the extension
office for verification if you like.

It is very common, and most gardeners face this problem with potatoes or
tomatoes some time or other.  It is worst on plants stressed by drought, insects
or nutritionally defeciencies.  It is aggravated by moist condition and lessens
with dry periods.  Don't overhead water and try to protect your plants from
extended rainy periods. Keeping the leaves dry will at least partially control
it.  Good ventilation helps.

It can infect the tubers too, causing dry brown spots that may go mushy as
secondary infections begin.  Mounding soil over the tubers, keeping them well
covered, will help prevent them from getting infected.

If not too much of you plant is effected you can try picking off the worst
leaves and removing them from your garden.  Be careful to wash hands and tools
after touching infected tissue, it can be spread by both of these.  Severly
effected plants should probably be destroyed.

Flea beetles can be a vector sometimes and the disease will appear after the
flea beetles have been around - they usually emerge pretty early in the summer
or late spring.

There is an organic copper fungicide spray you can use to control it, which you
apply at 7-10 intervals from first sign of the disease onwards.

The fungus overwinters in leftover crop debre, so be sure to clean up any
effected material at the end of the season.  You can dig a hole and bury it
safely in your garden or add it to a hot compost pile.

It's a good idea to rotate your crops on a 3-4 year cycle, avoiding planting
potatoes and tomatoes in an infected area for that long (if possible).

Planting resistant varieties in rich soil will help prevent the disease in
future.  While it is nearly impossible to completely eradicate - fungus spores
can travel some distance on the wind - you can keep it from becoming a crop
destroying disease with good cultural practices.

Good luck,

   ____________________
  |                    |
  |     Bob Carter     | Kootenay Bay, BC, Canada
  |  bcarter@awinc.com | Zone 6b
  |____________________|


... Handy Guide to Modern Science:
    1. If it's green or it wiggles, it's biology.
    2. If it stinks, it's chemistry.
    3. If it doesn't work, it's physics.

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