This article was first published in Potato Review, November 2008, p28 (http://www.potatoreview.com/)
If you look at some text book pictures of watery wound rot, you might think that this is a condition that is easily recognised. The tuber centre often appears grey and wet and the North American name shell rot accurately describes the way in which the rot hollows out the tuber, leaving the outer layer as a shell. However my impression of the disease changed radically when rapid detection kits became available. I found that many difficult to identify rots tested positive for Pythium spp., a group of moulds, one of which, Pythium ultimum, causes Watery Wound rot. I found that often these did not look at all like the text book pictures. One of the reasons we see these complications is because once a tuber starts to rot, other organisms which are lying dormant also become active. One of the principle culprits can be Pectobacterium atroseptica, which causes blackleg. There are several other bacteria which can cause tuber wet rot. The lessons to be learned from this are firstly that we need continuously to carry out tests to corroborate our observations and secondly that watery wound rot may be more widespread than we realise.
In North America, Pink Rot and Watery Wound Rot are often lumped together as “water rots”. This reflects a similarity between the symptoms and also the circumstances under which the diseases occur. Like Pink Rot, Watery Wound Rot is associated with warm weather and damage during harvest. However the critical temperature for Watery Wound Rot is a little higher than that for pink rot. Whereas to prevent Pink Rot, advice in the US is not to harvest until the temperature is below 18.5°C, for Watery Wound Rot this figure is 21°C. Unlike Pink Rot infection appears to be dependent on the presence of wounds. This has a consequence for fungicide protection. Metalaxyl is better at controlling Pink Rot than Watery Wound Rot. The explanation lies in the fact that much initial Pink Rot infection arises from invading zoospores prior to harvest. Most Watery Wound Rot appears to arise from soil contamination of wounds. In this way the disease appears to bypass the metalaxyl barrier. Like for Pink Rot, metalaxyl resistance has been found for Watery Wound Rot. Also like Pink rot there are variety differences offering scope for breeding watery wound rot resistant varieties in the future but there is little choice of variety to avoid the disease today.
Pythium ultimum, the organism which causes Watery Wound Rot, can cause damping-off of seedlings and rotting of roots, stems and fruits of wide range of crops. As far as we know there are no special strains which prefer one crop to another. It forms oospores, sporangia and zoospores and these may be instrumental in long term survival. However it appears to have developed its own special strategy for competing with other soil organisms by germinating whenever there are nutrients available in the soil and then retracting again to a form in which it can survive long term. This strategy may also be the key to manage the disease if only we could understand the positive and negative factors which affect this process. Some soils may simply be unkind to P. ultimum and if we understood why, we could exploit that.
Watery wound rot, on a scale that is reported in North America, is unlikely to be an issue in this country. However the number of cases where watery wound rot has been confirmed raises the suspicion that the organism which causes watery wound rot is around and may be causing more harm than we realise even at below threshold temperatures. Sorting out what has gone wrong, once you are confronted with serious rotting is a complicated business. Fortunately methods for identifying different causal organisms are improving all the time. For some time now there have been kits* which are specific for Pythium spp. and for Phytophthora spp.. This means that they can be used in combination to tell suspected pink rot from suspected watery wound rot, but will not tell the difference between Phytophthora infestans (blight) and Phytophthora erythroseptica (pink rot). The essential measures to manage watery wound rot are similar to those for pink rot. These measures consist generally of damage prevention at harvest and optimal conditions during the early storage period. Additionaly, when things go wrong it is well worth spending some money either on some test kits or on sending a sample to a reputable lab.
- Be suspicious of any wet rot and test for watery wound rot
- Ensure good skin set prior to harvest
- Minimise damage during harvest
- Remember that risk increases with higher temperatures at harvest (21°C is the recommended threshold for harvest in North America)
- Prevent infected tubers from going into store
- Ensure adequate curing to promote wound healing
- Tuber temperatures should be taken to below 10°C as soon as is practicable
- Prevent condensation in store