Symptoms and Management of Diseases, Pest damage, Nutrient Deficiencies and other problems of Macadamias in Southern Africa Updated February 2015

Transcription

1 Symptoms and Management of Diseases, Pest damage, Nutrient Deficiencies and other problems of Macadamias in Southern Africa Updated February 2015 Contents Symptoms and Management of Diseases, Pest damage, Nutrient Deficiencies and other problems of Macadamias in Southern Africa... 1 Introduction... 2 Iron Deficiency... 3 Magnesium Deficiency... 5 Phytophthora Trunk Canker... 6 Phytophthora Root Rot... 8 Trees Planted Too Deep / Newly Planted Trees Dying... 9 Branch Dieback Armillaria Dark patches on stems Husk Rot / Anthracnose Blossom Blight Rat Tail / Raceme Blight Spots on Leaves / Leaf Hopper Thrip Damage / Mite Damage / Bronze Nuts... 17

2 Introduction Due to macadamia being a relatively new crop in southern Africa, it is expected that new diseases will arise during the next decade or two. This could happen when pathogens of macadamia from other parts of the world are introduced to this area, or when pathogens on other crops in southern Africa perform what is called a host jump. Growers are urged to contact Subtrop if any suspicious new disease symptoms are observed. This document will serve as a guide to identify some of the known symptoms of diseases, pests and nutrient deficiencies that have been observed on macadamia in South Africa. 2

3 Iron Deficiency Symptoms: Leaves become a pale green-yellow and often develop necrotic edges. Often dark green spots are visible on leaves. Nuts can also become yellow. In severe cases tree canopies become sparse and some branches might die or appear dead. Photos: Barry Christie; Morné Pretorius 3

4 Cause: The symptoms are due to iron deficiency in leaves, however in practically all cases the underlying cause is not due to an iron deficiency in the soil. Usually the roots would be affected thus creating an imbalance between the root : canopy ratio. This condition is often related to waterlogged conditions, either caused by excess rain or over-irrigation. To complicate matters, waterlogging creates ideal conditions for oomycete pathogens such as Phytophthora and Pythium spp. to infect roots. Waterlogged conditions are often caused by shallow soils or compaction. Other causes could include too high phosphorous levels in soils, or too high or too low soil ph. Treatment: Determine the underlying cause and treat accordingly. Foliar Fe applications could reduce the severity to some extent, and would only be a temporary solution. Stem injections with iron has also proven to work. It could help by pruning trees to restore the balance between the canopy and roots. If soil compaction is the underlying cause ripping as close as possible to the main stem is recommended. Applying a thick mulch layer would be beneficial, but keep in mind that plant material does not contain high quantities of Fe and an iron chelate application would also be recommended. 4

5 Magnesium Deficiency Symptoms: Yellowing develop on leaf edges as well as between veins. The symptoms first develop in older leaves. Magnesium deficiency is frequently seen in macadamia orchards. Cause: A deficiency of magnesium in the plant leaves. This symptom is frequently seen in macadamia orchards. Photo: Barry Christie Treatment: Apply dolomite, magnesium oxide or any other relevant nutritional product according to a soil analysis and fertiliser programme. 5

6 Phytophthora Trunk Canker Photos: Barry Christie and Dr Barry Manicom. Symptoms: Cracks in stems of trees, young and old, often associated with an oozing of reddish plant 6

7 sap. Cause: The oomycete, Phytophthora cinnamomi. Possibly other Phytophthora species as well. Research funded by SAMAC has indicated that infection by certain Pythium spp can also result in symptoms as severe as P. cinnamomi. These pathogens reproduce in water. Micro sprinklers are the main source of inoculum when spraying contaminated dam or river water onto the tree trunk. Treatment: Avoid spraying contaminated water onto trees by either avoiding the water touching the irrigation water from reaching the stem, or by treating irrigation water with an environmentally friendly biocide. Currently no product is registered to treat trunk canker in macadamias. However, the disease can be treated and perhaps even cured by applying phosphonic acid / phosphonate / phosphite trunk paints. 400 SL solutions are commonly used by diluting 1:1 with water, and when 200 SL products are used no dilution is made. The use of penetrants will in all likelihood enhance the uptake of the product. 7

8 Phytophthora Root Rot Photos: Barry Christie Symptoms: Tree canopies become sparse and top branches shed leaves, often resembling drought symptoms. Some literature report that macadamia roots do not show any necrosis, however in severe cases root necrosis does occur. Cause: The oomycete pathogen Phytophthora cinnamomi. Treatment: Same treatment as for stem canker. Foliar applications of phosphonate products could also be performed, granted that there are leaves to take up the product. 8

9 Trees Planted Too Deep / Newly Planted Trees Dying Symptoms: Young trees appear to die for no apparent reason several weeks or even months after planting. When the bark at the base of the stem is peeled off a fungal growth is often visible. The phenomenon is particularly common in the A4 cultivar. Photos: Barry Christie Cause: unknown possibly several species of fungi. Treatment: There is no known treatment. Try to avoid planting trees too deep. Remove mulch from the base of the tree so that there is no direct contact to avoid the onset of collar rot. 9

10 Branch Dieback Symptoms: Individual branches start to die back, often more than one branch per tree. Leaves become brown as a result of dead tissue, but leaves remain on the tree. Over a matter of several weeks or months the whole tree could be affected and die. Photo: Dr. Barry Manicom Cause: Various pathogens within the Botryosphaeriaceae family of fungi. These include Botryosphaeria spp, Dothiorella spp. Neofusicoccum spp of which Neofusicoccum parva and N. ribis is the most commonly associated with this disease. These fungi live endophytically in plants without causing disease and upon the onset of stress - most often drought stress - switch over to a pathogenic phase. Treatment: There is no known effective chemical treatment. It is crucial to determine the root cause and relieve the underlying stress. Cut off and destroy dead branches. 10

11 Armillaria Symptoms: Tree canopies develop a rapid fire-damage-like symptom. Armillaria spp. can usually be identified by a typical tree-like structured fungal growth often below the bark at the base of the tree is or below the outer layer of major roots. Cause: Fungi within the genus Armillaria. The disease is often prevalent in orchards previously planted with susceptible hosts, for instance avocado. Treatment: Treatment is challenging. It is recommended to remove old stumps before planting macadamia trees in an orchard. Infected trees should be removed as soon as possible. It is advisable to dig a trench of at least 40cm deep around infected trees to avoid root contact with neighbouring trees. After removing trees it is recommended to leave the hole bare and exposed for at least six months. Fumigating with metham sodium can also be considered. Although not registered, injecting or spraying trees with propiconazole has proven to be effective in several tree species. 11

12 Dark patches on stems Symptoms: Dark, blackish fungal patches on stems with almost the resemblance of lichens. Photos: Andrew Sheard Cause: The fungus Phanerochaete crassa has been identified as a causal organism of these symptoms in KwaZulu-Natal. Little is known about these fungi and the economic importance this disease is also yet to be determined. In other plant species Phanerochaete spp. are not known to cause disease and these fungi grow superficially on the bark and dead plant material. However, browning of cambium has been observed. Fungi within this genus generally grow in humid, warm conditions. Control: There are no known control methods since little is known about this disease / fungus. 12

13 Husk Rot / Anthracnose Photo: Dr. Barry Manicom Symptoms: Macadamia husks develop black lesions. The lesions do not have clear, finite edges and often appear to merge with the green colour of the husk. These lesions can spread onto the nut. Husk rot can cause premature nut drop. Cause: Anthracnose is caused by the fungus Colletotrichum gloeosporioides, which is a fungus found frequently on many other subtropical fruit. Phomopsis spp. has also been isolated in South Africa on diseased nuts. Treatment: Amistar Top (Syngenta) and Bellis (BASF) will both control the disease. Reduce inoculum by performing good orchard hygiene, for instance remove old nuts from the orchard floor and husks should be composted before using as a mulch. 13

14 Blossom Blight Symptoms: Individual flowers on the raceme become brownish until a whole raceme is covered with dead flowers. The flowers will eventually become mouldy as a greyish greenish fungus becomes visible. Photos: Barry Christie; Macadamia Problem Solver & Bug Identifier, Queensland Government, Australia. Cause: The fungus Botrytis cinerea has been suspected of this disease in Australia. Although B. cinerea is present in South Africa, this disease has not been readily observed. Phytophthora nicotianae has also been identified as a causal organism of blossom blight in South Africa. Treatment: The systemic fungicides Bellis and Amistar Top should control Botrytis. No chemicals are registered to control Phytophthora blight on macadamia. 14

15 Rat Tail / Raceme Blight Photos: Dr Barry Manicom; unknown Symptoms: The bottom / tip of the raceme becomes necrotic, which spreads upwards. As the symptoms progress, more flowers drop and the raceme stalk becomes bare, hence the term rat tail. The disease develops in cool, humid conditions. Cause: The fungus Cladosporium cladosporioides. The disease is often mistakenly regarded as Botrytis infection, but after extensive research funded by SAMAC, Botrytis spp. has not been isolated from diseased macadamia inflorescences. Treatment: The systemic fungicides Bellis and Amistar Top should control the disease. Be alert of the disease during cool, wet and humid conditions, which favour disease development. 15

16 Spots on Leaves / Leaf Hopper Photos: Barry Christie Symptoms: Light yellow circular patches appear on leaves. Later, a light red or pinkish dot appears in the centre of the patch and the patches become more prevalent and darker in colour. Cause: Although it has not been proven yet, it is thought that the symptoms are caused be a leaf hopper, an insect with pierce-sucking mouth parts. Control: For most growers the symptoms should not have an economic impact which warrants control. However, in nurseries these insects can cause severe damage which not only impacts the appearance of the plants, but could in severe cases reduce the plant s growth rate due to loss in photosynthetic surface area. Systemic neonicotinoid insecticides such as Actara or Kohinor controls these pests. 16

17 Thrip Damage / Mite Damage / Bronze Nuts Symptoms: Husks become brown or bronze in colour. The damage to nuts is cosmetic since husks will be removed. Young leaf flushes can be stunted often a problem in nurseries. It is also suspected that flowers are attacked. Photos: Barry Christie; Dr Schalk Schoeman 17

18 Cause: Thrips and/or mites. These insects are usually the first to build up resistance against insecticides currently the extensive use of pyrethroids cause population spikes of thrips, presumably due to the removal of its natural enemies. Treatment: Avoid extensive spraying with insecticides by relying on timeous spraying based on scout results (specifically referring to spraying for stink bugs). The economic threshold of controlling these insects is difficult to determine. Scout regularly for thrips, mites and their damage and keep records. Work with trends rather than absolute numbers. When necessary, the appropriate miticide or insecticide can be used. The use of insecticides on flowers is generally not recommended since bees need to be active for optimal pollination. 18