Sudden Wilting of Healthy Kinnow Trees: Probable Causes, Early Identification and Tips for Management

J S Brar

and Krishan Kumar

In recent years, a problem of sudden wilting followed by drying of Kinnow trees has been witnessed in the South-Western region of Punjab, parts of Haryana, and Rajasthan. Apparently, healthy trees show symptoms of sudden wilting and then turn dead within a few days. Though, this problem was observed in some orchards in the past as well, in the last couple of years its incidence is increasing. The Punjab Agricultural University has tried to find out the possible reasons for this problem, early diagnostic symptoms of this malady, and possible remedial measures. The exact cause of the sudden wilting of healthy trees is not certain yet, but the findings suggest the imbalanced root-shoot ratio as the probable cause behind it. Primary investigations show that due to certain biotic and biotic factors such as high temperature, high-water table, high salt concentrations in the root zone, Phytophthora infection, soil nutritional imbalance, heavy fruit loads, indiscriminate use of agro-chemicals, etc. leads to the weakening of plants.

The root system of the affected plants is pre-disposed to attack by soil-borne pathogens that cause the rotting of fibrous roots. Under the conditions of high temperature and low relative humidity, evapotranspiration loss from plants occurs at a higher rate and requires an increased demand for water uptake from soil. The roots of the affected plants are incapable to supply the required amount of water and nutrients and eventually tree starts wilting. In this article, the early diagnostic symptoms of wilting plants and their possible management strategies have been suggested.

EARLY DIAGNOSTIC SYMPTOMS OF AFFECTED PLANTS

• Excessive flowering and fruiting
• Reduction or complete loss of glossiness in leaves of plants
• Slight drooping of leaves
• Fruits on the affected trees remain small and become soft at the advanced wilting stage
• Rotting of fibrous roots

MANAGEMENT

A. Measures to prevent the onset of the wilting

i. Balanced application of fertilizer and organic manure

Kinnow is a heavy yielder and thus, is also a heavy feeder. A balanced application of fertilizers and organic manures is necessary to replenish the nutrients used up by the fruits. But, in recent years, it has been observed that the farmers supply mainly nitrogenous and phosphatic fertilizers along with 1-2 sprays of micronutrients (mainly for correcting Zn and Mn deficiencies). For balanced growth and yield, plants synthesize various metabolites. For the synthesis of these metabolites, the plants require different macro and micronutrients. Organic manures (Farm yard manures, FYM) are a rich source of various macro and micronutrients. The FYM also creates a favorable environment for the absorption and availability of major nutrients and for the thriving of beneficial microflora. An adequate amount of organic manure improves the structure of the soil and helps in diluting the impacts of high salts in the rhizosphere of the tree. In juvenile orchards, green manuring can also be practiced to enrich soil organic matter.  Further, the imbalanced application of one nutrient may disrupt the availability of the other nutrient. To avoid this, fertilizers should be applied as per the soil and plant requirement. To estimate the exact nutritional requirement of the plants, the soil should get tested. Soil samples should be collected underneath the tree canopy at the site of the maximum density of the feeder roots. Looking at the fertilizer application pattern of the growers, the soils can be examined in November-December before the application of organic/ inorganic fertilizers.

ii. Use good quality irrigation water and conserve soil moisture

In South Western Punjab, the groundwater is generally of brackish nature. Growers sometimes use underground water to save their orchards during canal closures. Kinnow and other citrus fruits are highly sensitive to salinity, thus only applying water of good quality. Under hot weather conditions, the application of brackish water creates a stressful environment for the tree. If the tree roots are already injured, such water proves very harmful. It is also obvious that the plants are in urgent need of water during the summer months. The storage of canal water in storage tanks can be a good option. In hot months, the loss of water occurs both from plants (transpiration) and soil surfaces (evaporation). The transpiration creates a pull force necessary to translocate the nutrients and water from the soil to different parts of the plant. On the other hand, evaporation causes loss of water in the hot months. To prevent loss of water through evaporation, organic mulches (like paddy straw mulch, grass mulch) can be applied. This practice not only helps in conserving soil moisture but also suppresses weed growth.

iii. Adopt conservation tillage

In Kinnow, most of the feeder roots lie in the top 15-20 cm. During frequent tillage, these roots are damaged, which not only impairs the water and nutrient translocating capacity of the plant but also predisposes them to various soil-borne fungi including wilt-causing pathogens. Hence avoiding frequent tillage can prevent injuries to the roots.

iv. Adoptpruning to manage tree workload

Sometimes young plants of 4-5 years produce a large number of fruits. The large number of fruits at this tender age causes stunted growth and sometimes tends to decline. Limit the fruits at this age and thin out the excess fruits to prevent the premature decline of the young plants. Sometimes older trees also carry exceptionally high fruit yields due to favorable weather support. We saw this type of bearing in 2020 and even in the current year when exceptionally low temperatures prevailed during the fruit set and initial fruit development phase which prevented the early fruit crop. This proved one of the pre-disposing factors for the tree to decline in the same year or in the subsequent years. Thus, for increasing tree life, fruit in excess of tree-bearing capacity can be shed off as and when noticed. Regulation of flowering by adjusting the pruning is also one of the best options in this regard.

v. Timely manage insect pests and diseases

Manage insect pests and diseases as these weaken the plant's defense system and affect the plant's growth and yielding capacity.

B. MANAGEMENT OF WILTING TREES

• Profusely flowering plants should be monitored regularly and reduce the fruit load if early symptoms of wilting appeared. Reduced fruit load will stabilize the wilting plants.
• The plants showing loss of glossiness of leaves or moderate wilting symptoms should be pruned to the extent of 20-30 percent to balance the root: shoot ratio. Reduced leaf area will reduce the evapotranspiration loss of water. Preferably, remove thin twigs instead of thick branches.
• The rhizosphere of the wilting plants has been found to show fungal infection. Use appropriate fungicides in consultation with the experts.

The growers should adopt balanced management approaches to prevent the onset of wilting. Early identification of wilting plants and adoption of management practices can save the plants from quick decline. The scientists working in the Department of Fruit Science, Punjab Agricultural University Ludhiana, and Dr. JC Bakhshi Regional Research station Abohar may be contacted for further information in this regard.