Role of urea spray in reducing apple scab

Fungicides are the primary tools used to manage apple diseases. But fungicides have limitations that may reduce their efficacy, requiring the addition of other strategies such as sanitation for complete disease control. Sanitation practices remove the overwintering stage of the pathogen and thereby decrease the primary inoculum, which ultimately reduces fungicide applications required during the growing season. This may be especially beneficial in seasons where disease pressure is so high that fungicides alone are not effective. There have been several studies exploring the reduction of initial inoculum of V. inaequalis by implementing sanitation practices. Since the pathogen overwinters in leaves on the orchard floor, these practices focus on eradicating the leaf litter by removal, shredding, chemical treatments, or urea applications. Shredding the leaf litter can reduce scab incidence by 90% in a controlled environment. The reduction of inoculum from shredding or removal of the leaf litter may range from 50-65% due the variable topography and obstructions in true orchards.  Alternatively spray applications to eradicate leaf litter may be an effective substitute to shredding and removal. Chemical compounds such as mercury in the past have shown promising results in reducing primary inoculums, and the number of fungicide treatments required during the growing season. But due to their harmful effects to the environment and other organisms, many of the chemicals used in these studies have been taken off from the market. This has led to research of other treatments using compounds that are more acceptable by today’s safety standards. Among these compounds urea spray applications are found to be the most effective at reducing overwintering inoculums.

Urea

Urea is a synthetic crystalline solid containing 46% nitrogen. The high nitrogen content, low price and ease of handling and storage has made urea an ideal source of nitrogen for agriculture over other dry nitrogen sources. However, because urea is synthesized it is not approved for organic agriculture. So the benefits of using urea can only be realized in conventional agriculture. In conventional orchards, one use of urea in agriculture is to apply it to apple leaves to enhance litter decomposition by promoting growth of saprophytic microorganisms in the soil. Enhanced decomposition of the leaves decreases substrate available to V. Inaequalis needed to develop overwintering inoculum in the spring. The high nitrogen content in urea also directly inhibits pseudothecia or fruiting body and development of V. inaequalis. V. inaequalis requires depletion of all nitrogen before pseudothecia development can occur and excess nitrogen can delay or prevent this next step in the lifecycle. Urea applications have also been found to inhibit pseudothecia development by stimulating bacteria that is antagonistic to V. inaequalis. Urea is applied post-harvest before or after leaf-fall in early autumn, preventing the overwintering stage of V. inaequalis. Urea can be applied to the orchard floor after leaves have been abscised from the tree. Issues with post leaf-fall applications include inadequate coverage of the leaf surfaces and reduced treatment efficacy. Furthermore, ground applications may be insufficient in reducing inoculum if the ground freezes soon after the urea treatment is made. This is because low temperatures slow the urea-driven decomposition rate of the leaf litter. These facts outline the limitations of urea applications, but they also reveal a potential area of research. Such improvements would enhance urea-driven leaf litter decomposition and thus further reduce initial inoculum of V. inaequalis. Too few studies have looked at remedying the limitations of the ground applications, as they are safer regarding tree health. Furthermore, to date there has been no research to explore compounds that increase the coverage and penetration of urea from a sanitation standpoint. Increasing urea treatment efficacy may result in a lower amount of primary inoculum in conventional and organic orchards which may potentially reduce the amount of fungicides needed to control apple scab during the growing season. With respect to urea application our university SKUAST-KASHMIR has issued an advisory to spray 3-4% urea when 15-20% yellowing of leaves or leaf fall has occurred. This will interrupt the life cycle of apple scab pathogen by hastening the decomposition of apple leaf.

Dr Shabeer Ahmad Ganie is Scientist Plant Protection KVK Shopian SKUAST-KASHMIR