07.2014 Green molds and mushroom feeding

Green molds are the most important pathogens infesting substrate in the mushroom production. Their presence and the development of their colonies still present the potential cause of the most serious losses. I have been interested in this problem since early 2000. In Poland, the highest losses resulting from green molds infections occurred in the years 2002 – 2009. In 2009, I published the book “Green molds in mushroom production” (PWRiL) regarding this topic.

Presently, two genera of fungi, which are considered the green molds, are described as the main causes of losses in mushroom production. The highest losses result from infections by the genus Trichoderma, particularly by Trichoderma aggressivum. The recent data indicate potential serious threats from other pathogen Penicillium hermansii (smoky mold) Hermans C., Houbraken J., Smokey Mould: the smoke screen lifts, Mushroom Business, 061 November 2013. Both these species (strains) share one feature i.e. they are considered the aggressive mushroom pathogens.

How can one characterize a current concept of losses caused by the green molds that develop in the compost and infect mushrooms?

This theory assumes the existence of a correlation between the presence of spores of pathogens such as Trichoderma aggressivum and other species of Trichoderma spp. with competitive behavior and the smoky mold (Penicillium hermansii) in the compost, and also the development of their colonies as the result of infections with spores or mycelium, and destabilizing the selectivity of compost. This hypothesis can also be illustrated in other words i.e. colony size might be larger if more spores of pathogens survive during the compost production process and the compost is less selective. The losses resulting from the primary infection are the most severe particularly if colony development occurs in the tunnel. Whereas the secondary infections that happen between the completion of compost phase II production and at spawning until applying casing cause much less losses. It means the early and severe infections while the compost is less selective results in higher losses.

Despite numerous scientific studies there are no satisfactory results that would help solve the problem regarding how to protect the compost against infections of mentioned above pathogens. This situation becomes more difficult as a current problem of losses caused by Trichoderma aggressivum disappears in itself and the smoky mold does not indicate an increasing threat. There is no information regarding new green mold infections. Personally I have seen the infection with smoky mold several times over 20 years of my consulting practice. For instance, in the past the smoky mold infections were observed occasionally and did not cause significant losses. They are not perceived in many countries with the mushroom production. Regarding this situation one can ask the question if this problem solved itself and forever? Will we experience new infections in the coming years? If the infections do occur, how do we prevent the losses? So far there is no satisfactory answer.

Preparations of the concept regarding development of mushroom production technology as controlled feeding and its implementation requires additional review of this issue. Solving the problem of yield losses caused by the green molds considers two potential possible approaches.

  1. The first approach would exclude using compost in the mushroom production that would eliminate primary infections. Instead of the compost it is recommended to apply a substrate within control of its microbiological environment; lack of primary infections and protection against secondary infections by the simultaneous introduction of mycelium and casing application.
  2. The second approach considers changes in a feeding process that would protect the mushrooms against secondary infections and minor primary infections. In this case the compost plays the secondary role in a mushroom feeding. Properly composed feeders create conditions for full control of microbial composition of compost during its recolonization after their addition into a substrate phase III. The mycelium will become so strong that it will not allow pathogens to develop and compete for nutrients. The nutrient competition and pathogen presence are a main cause of losses in mushroom production. This means necessity to provide a surplus of mushroom mycelium during the recolonization process and during feeding after the casing application that a minor primary infection will not occur, and in consequences secondary will not take place either at a stage of placing on the shelves. This is a significant advantage regarding dominance over nutrients and competitors that might be present in the compost. This proceeding should be efficient enough. This approach is based on two assumptions that are accepted as legitimate. Although the genus Trichoderma and Penicillium are the competing species and more opportunistic towards food source that the mushroom, they show different food preferences. They utilize protein better than the mushroom, suggesting that there should be lower protein content in mushroom feeding. The second approach assumes that these species are not aggressive and they become destructive only in certain environmental conditions, and this aggressiveness is transmitted into another environment via vegetative way. Aggressive behavior occurrence among competing species has already been reported and it is not very rare phenomenon. The question is what causes this aggressive behavior. The following factors might be the deciding elements: colony size of pathogen, disruption in compost selectivity and process of compost colonization by mycelium. What is the reason that aggressive species and aggressive behavior have been discussed? It results from the fact that I have never observed secondary infections with Trichoderma aggressivum in places that had prior infections. It confirms that secondary infections do not occur in production facilities with high hygiene procedures that include steaming after compost production and also in facilities with very low hygiene without steaming measures. These observations refer to hundreds of reported cases. In contrast the losses caused by dry bubble disease (or brown spot) show a clear correlation between hygiene practice and the level of colony development of this pathogen. Performed observations indicate that a diet based on high polysaccharidecontent results in very rigorous expansion of fungous in the compost and that inhibits Trichoderma and Penicillium infections that might take place during the placing of substrate phase III on shelves.

I am interested in both solutions.

However, the pathogens might colonize the supplements and cause significant losses in production if these supplements based of vegetable origin, such as ground corn, are improperly prepared and stored.

A separate aspect is a potential mushroom strain that would be resistant to the green mold present in the compost. In my opinion, it is difficult to count on such a solution mainly due to the difficulties of identification of genes that should be modified to obtain such a resistance. Besides, finding these genes is one problem and another one is an implementation of genetically modified mushrooms into production and acceptance among producers. Presently the resistance can only be achieved in the genetic modified material and that also requires funds and executor. The current lack of real threat to mushroom production yield makes this issue of little interest among mushroomproducers.