Rehabilitation of Asia's Citrus Orchards
Citrus orchards in Asia are suffering heavy losses from virus diseases and citrus greening
Citrus Greening
Citrus greening is the most common and the most destructive disease of citrus orchards in Asia. It is transmitted by tiny insects called psyllids, which suck the sap from young leaves. When a psyllid feeds on an infected tree and then moves on to a healthy one, it often carries the greening pathogen with it.
Fortunately, the psyllids found in Asia cannot tolerate cold weather. Korea and the main islands of Japan are too cool for the Asian citrus psyllid, although it is found in Okinawa, the subtropical Japanese island. However, in both Japan and Korea there is some concern about the possible expansion of greening from the south, as the result of mutant strains and global warming.
In tropical Asian countries, the rate of infection by greening is often more than 50%, and may reach 70%. As a result, average yields have been falling. For example, the average citrus yield in Vietnam's Nghe An Province during the 1960s was 18-20 mt/ha. Today, average yields are only around 6-9 mt/ha. Similarly, the life span of orchards has fallen from 17-18 years in the 1960s, to less than a third of that today. The Nghe An Provincial Government is now carrying out a strong program to control citrus greening .
Unfortunately, it has not been possible to breed resistant varieties. Most citrus cultivars are susceptible to greening disease. Pomelo was thought to be resistant, but in recent years some pomelo trees growing in Asia have been found to be infected.
Common symptoms of citrus greening are yellowing of the veins of the leaves and adjacent tissues, followed by yellowing or mottling of the entire leaf. This is followed by dieback of twigs, a decline in vigor, and eventually the death of the tree.
Control of Citrus Greening
Control of this disease is based on control of the psyllids which transmit it. The best way to do this is by well-timed chemical sprays when psyllid populations begin to rise. Psyllids prefer to feed on young shoots. Growers must monitor psyllid numbers, and spray new leaf growth as soon as the psyllid population begins to increase.
Often greening disease is spread via infected planting stock. Trees used as foundation stock must be free of citrus greening and virus diseases. Indexing is essential to ensure this. Plant nurseries can then produce certified disease-free seedlings for distribution to growers.
Tristeza
Almost 90% of citrus trees in Asia are infected with tristeza, a virus transmitted by several species of aphid. Many strains are mild ones which do little damage to the plant. However, some new virulent strains have appeared in recent years which are a serious threat to citrus production.
The symptoms of tristeza vary according to the strain of the virus, and the variety of citrus. Like greening disease, tristeza is difficult to diagnose from the visible symptoms alone. In fact, many trees in tropical Asia suffer from both greening and tristeza. In the past, the main method of diagnosing tristeza virus was to inoculate an indicator plant, usually Mexican lime (Citrus aurantifolia). Now we have a new test kit which can be used in the field and gives results in a few minutes.
Control of Tristeza
The main method of control is the use of resistant rootstock such as mandarin or trifoliate orange. Pre-immunization can sometimes be useful. This is a technique of inoculating virus-free citrus plants with mild strains of the tristeza virus, in order to protect them from severe strains.
Rehabilitation of Citrus Orchards
Greening disease and virus diseases can only be controlled if new trees planted out in the field are free of disease. This needs a national system of healthy foundation stock, which can be used to produce certified disease-free planting materials.
The next step is to keep the young trees healthy once they are planted out in the field. This should follow an integrated approach that begins with the orchard design, and continues with good soil and water management. Constant monitoring allows growers to apply prophylactic sprays at an early stage of pest build-up, according to integrated pest management (IPM).
Success must be seen in terms of the life span of the orchards, not in the impossible goal of 100% healthy trees!
Disease-Free Citrus Plants
Disease-free citrus seedlings for growers depend on a supply of healthy foundation stock, to provide budwood for grafting. The People's Committee of Nghe An Province in Vietnam has built thirteen insect-proof screenhouses for disease-free foundation stock. These include traditional varieties, as well as Valencia orange.
Next to the screenhouses is a demonstration orchard, where growers are shown how to manage their healthy trees so as to keep them free of infection.
The rootstock are grown from seed, which breaks the cycle of virus transmission. Budwood is taken from the foundation stock and grafted onto the rootstock. FFTC has held several training courses on shoot-tip micrografting. This is a very efficient way of using foundation stock, since one mother tree can provide enough buds each year for thousands of grafted plants.
Index of Images
Figure 1 Citrus Tree with Phytophthera Root Rot Growing in Poorly Drained Soil
Figure 2 Five-Year-Old Citrus Trees with Advanced Greening Disease
Figure 3 Screenhouses in Nghe an Province, Vietnam, for the Production of Disease-Free Citrus Seedlings
