Plant disease?

>From: Erik Olson <olson at phys_washington.edu>
>> From: Hardjono.Harjadi at Eng_Sun.COM (Hardjono Harjadi)
>> 	Some of my plants (Hygrophila difformis and Hygrophila stricta)
>>  start developing the following disease. The plant leaves are still green 
>> but there are quite a few small yellow dots with black centers on them. 
>> After a while, the black dot gets larger and leaves a hole in the middle.
>> Any idea what kind of disease is this ? All of the other plants in the tank
>> (Ludwigia, water sprite, bacopa, anubias, riccia fluitans, rotala 
>> rotundifolia) are doing great.
>Snail damage, maybe?  I see similar effects on some plants in 
>snail-infested tanks.

I occasionaly see this problem in my snail-free tank as well.  All
the books say that this symptom is typical of snail damage, but maybe
my Otocinclus are to blame.