Powdery scab

Powdery scab
Scientific classification
(unranked):	SAR
(unranked):	Rhizaria
Phylum:	Cercozoa
Class:	Phytomyxea
Order:	Plasmodiophorales
Family:	Plasmodiophoraceae
Genus:	Spongospora
Species:	S. subterranea
Binomial name
Spongospora subterranea (Wallr.) Lagerh. 1892

Powdery scab is a disease of potato tubers. It is caused by the cercozoan Spongospora subterranea f. sp. subterranea and is widespread in potato growing countries.Symptoms of powdery scab include small lesions in the early stages of the disease, progressing to raised pustules containing a powdery mass. These can eventually rupture within the tuber periderm. The powdery pustules contain resting spores that release anisokont zoospores (asexual spore with two unequal length flagella) to infect the root hairs of potatoes or tomatoes. Powdery scab is a cosmetic defect on tubers, which can result in the rejection of these potatoes. Potatoes which have been infected can be peeled to remove the infected skin and the remaining inside of the potato can be cooked and eaten.

In general, not a lot is known about the life cycle of Spongospora subterranea f.sp subterranea (Sss). Most of the currently-proposed life cycle is based on that of Plasmodiophora brassicae, a closely related and better-studied protozoan. It has been proposed, due to this similarity, that there are two distinct stages that Sss can exist as; the asexual and sexual stages.

Asexual Stage: A zoospore infects root tissue and becomes an uninucleate plasmodium. This plasmodium undergoes mitotic nuclear division (creates many nucli within a single cell) and turns into a multinucleate plasmodium. Then, the multinucleate plasmodium forms zoosporangium, which eventually release more zoospores. This process can happen relatively quickly and can act as an important source of secondary inoculum within a field.

Sexual Stage: This stage follows a similar pattern to the asexual stage, but with a few exceptions. It is hypothesized that two zoospores fuse together to form a dikaryotic zoospore (with two separate haploid nuclei, n+n) and then infect the roots. Once the infection occurs, the dikaryotic zoospore develops into a binucleate plasmodium (one pair on nuclei, n+n). Similar to the asexual stage, this plasmodium will also replicate its nucleus to create a multinucleate plasmodium (many pairs of nuclei, n+n). The second main different between stages occurs here. The pairs of nuclei (n+n) will fuse by karyogamy, and the plasmodium will quickly divide into numerous resting spores within a sporosori (spore sack, alternatively called cystosori). These resting spores have three-layered walls and are extremely resistant to the environment, allowing them to persist in the soil for longer than 10 years.

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Wikipedia