Discovering Pythium and how it influences agricultural crops
Pythium is an oomycete, and even though oomycetes were previously considered a fungi, it has been recently shown that oomycetes are actually closer related to golden algae, brown algae, yellow-green algae and diatoms (read more). However, it is still common to find Pythium species considered as fungi or “fungus-like organisms”, as they share some basic functional characteristics. Other oomycetal genera which are popular include well known plant pathogens, such as Phytophthora and Saprolegnia, and all of these genera can cause dramatic root and seedling rots. In fact, Pythium and Phytophthora alone account collectively to multibillion dollar losses of crops worldwide.
Are all Pythium species able to cause plant diseases?
Some of the species of the genus Pythium are notorious plant pathogens, as some species can cause devastating rots in roots and lower stems of plants, as well as damping off in many vegetables and ornamental seedlings and young plants. Some of the species to blame for damping off and seedling rots include Pythium ultimum, Pythium aphanidermatum, Pythium disotochum, Pyhtium sylvaticum and Pythium irregulare (Source: Van der Plaats-Niterink, 1981; Rangaswami, 1962; Daughtrey & Chase, 1992). These aggressive Pythium species can also result in economic losses by stunting and slowing the growth, because they weaken the root systems of the plants. This can impact significantly on the profit margins of protected crops. As an example in cut flowers, Pettit estimated a 8-10% yield loss of infected chrysanthemums compared to healthy uninfected controls (Source: AHDB Horticulture Project PC97a). More significantly was the effect of Pythium in Kenya and Rwanda, where yield losses in commercial bean cultivars due to Pythium infection of seedlings in 2012 were reported to be up to 70% of the crop yield (Read more).
Figure 1. Fungicide Tolerance Study carried out by FungiAlert, which shows the susceptibility of different Pythium species to a widely used fungicide
Another well-known disease caused by Pythium which is responsible for high yield losses in the UK, is cavity spots of carrots. An AHDB study reported yield losses of up to 64% in untreated carrot fields (Source: AHDB Project FV-05g). One of the species identified as causing cavity spot is Pythium violae, however during field trials in 2017, FungiAlert’s SporSenZ identified a number of other Pythium species which are known to cause cavity spot, other than P. violae. Interestingly, the species found had different sensitivities towards the chemical fungicide which was used for preventing cavity spot in carrots. This is an extremely important discovery, as identifying other causing agents of cavity spot and understanding their sensitivity to crop protection products, could dramatically reduce losses associated with cavity spot within UK carrot crops.
To illustrate the reputation of Pythium as the crop destroyer, a report published in 2015 asked the scientific community to rank the plant pathogens based on scientific and economic importance, and Pythium ultimum was shortlisted amongst the 10 most relevant plant pathogens (read more). This one species of Pythium causes damping off and root rot on over 300 diverse host, including the top world food crops like soybean and wheat. P.ultimum produces oospores that can survive for many years in the soil and can become infective when the right environmental conditions happen. Disease management of Pythium ultimum is challengingand often involves sanitation, chemical and biological fungicide applications.
Nevertheless, it appears that plant pathogenic Pythiums may have a role in nature. Interestingly, it has been suggested that pathogenic Pythiums may play a significant role in maintaining the biodiversity of forest tree flora. This is because Pythium seedling disease prevent new trees from growing close to their parent trees (Packer & Clay 2000 & 2003; Van der Putten, 2000).
How does disease occur?
It is likely that Pythium pathogenic species are attracted to the root elongation zone of the adventitious roots in the juvenile tissues around the growing tips. When they enter the root tissues the pathogen causes a rapid rot, which in aggressive infections can rapidly extend and affect the entire roots, progressing to the lower stem and crown tissue (Read more) . Ultimately the plant will manifest symptoms of disease such as wilting, plant collapse and yellowing of shoots.
Are all the species of Pythium plant pathogens?
Interestingly, some species of Pythium may be more helpful to us than expected! Several species of Pythium clade D are mycophagus, or in other words, they attack and feed upon soil fungi and oomycete mycelium (read more). These species include P. periplocum, P. acanthicum and P. oligandrum. Pythium oligandrum is a mycoparasitic oomycete, capable of controlling the spread of other microbes that can cause diseases, like Fusarium, Phytophthora, Botrytis, etc.Besides attacking disease causing agents, it is believed that Pythium oligandrum can also establish symbiosis with the plant root system, which stimulates plant growth and triggers plant defence reactions (read more). Interestingly, FungiAlert’s soil health analyses carried out during the last couple of years found in healthy and rich British soils, the natural occurring source of biological control of pathogenic soil microbes, Pythium oligandrum. This is the first time that healthy and continuously well performing soils have been linked to the presence of Pythium oligandrum in British soils. The detection of beneficial microorganisms in the soil by FungiAlert’s SporSenZ reveals the potential of our soil analysis as a key tool to help selecting land for farming.
How can Pythium be controlled in fields?
Fungicide applications can act as protectants against Pythium outbreaks, helping to prevent infection rather than curing disease symptoms. These applications will only be efficient if used in advance or at the very early stages of disease. However, due to the limited research in horticultural crops about the effectiveness of fungicides targeting the soil root interacting pathogens, their application results are often fruitless, as seen in Figure 1, if the spray timings are not guided by early detection tools. By using FungiAlert’s sensors, disease outbreaks can be monitored and identified at very early stages. Therefore, our soil health analysis is extremely useful to identify the best timing for fungicide applications.
Due to the decreasing chemical controls available for root diseases caused by oomycetes, the use of biological control methods are becoming more important. For instance, the use of Prestop (Gliocladium catenulatum strain J1446) is approved in the UK and is recommended for the control of damping-off and root diseases caused by Pythium species. Unfortunately, due to the high complexity of the environmental activity surrounding the plant roots, biological controls can often show inconsistent success rates. This lack of reproducibility in treating plant diseases has made the uptake of biological control products by growers challenging. Similarly to the fungicide applications mentioned earlier, FungiAlert’s soil health analysis facilitates the best strategy for testing the effectiveness of biocontrol products, and helps to guide the application of the products to prevent disease outbreaks in crops.
The scientific community agree that if early detection and diagnosis methods were to be implemented routinely, economic losses resulting from disease development would be dramatically reduced. Early detection of disease pressures would allow the grower to optimise their crop rotation patterns, select resistant varieties, decide appropriate control measures and plan efficient harvest and post-harvest processes. However, early detection and diagnosis is not a general practice, because until FungiAlert introduced their technology into the market, there was no simple and efficient solution. To date, the only technology that allows for an efficient early detection of the infectious pathogens present in the soil of a field is the soil health analysis offered by FungiAlert, which is facilitated by their patented SporSenZ sensors.
Angela de Manzanos