Guava Wilt Disease
Guava Wilt Disease is a sudden, catastrophic killer of guava trees. The Institute for Tropical & Subtropical Crops has had a research program to try to develop methods of saving the guava industry in South Africa.
I don't know if guavas are an unusual fruit in Australia. They are not an unusual fruit in South Africa. In fact, in some places they have been declared weeds. If they are not grown in orchards, then you have to eradicate them from your land.
It is a commercial crop in South Africa. It used to be on a larger scale, but because of problems with diseases, it is now much smaller. There are two areas where it is grown: in the Western Cape, a winter rainfall area, and the Northern Province. The whole guava industry was based on one cultivar that was originally selected in the Western Cape, and then later brought into the Northern Province.
Guava Wilt Disease was first seen in 1981 and occurred in the Northern Province close to the Mozambique border. It spread rapidly through all the growing areas in the northern and eastern parts of South Africa. It is not found in the Western Cape. It had a big effect on production. The production of the Western Cape has fallen slightly, but that is due to plantings being removed for development. The Northern Province used to produce about 16,000 tons but by 1993 it had fallen to 6,700 tons of guavas per year.
The main market for guavas in South Africa is the processing market. The Western Cape does most of the canning, and the Northern Province does juice concentrates, a percentage of which is exported. The fresh market is very small.
Guava Wilt Disease killed off orchard trees randomly, and at the end of the day, there would be no orchard left. The original research concentrated on trying to control the disease with chemicals, but was totally ineffective. We did not find any chemicals to control the disease in any way. Symptoms include wilting of the leaves from the top of the tree. Sometimes it is a rapid decline and sometimes it is a slow decline. There is general wilting of all the leaves, chlorosis, the leaves become yellow or purple and drop. During the initial stages, the leaves dry out and often remain on the tree. If there are fruit on the tree, fruit development ceases immediately, and you get mummified fruit on the tree. In advanced stages, you get blistering on the stem, and then the blisters open and release spores into the air. It is a soil-borne fungus. It has changed genus every year--the taxonomists get hold of it. It is now Pinucillium?? but has had lots of other names. It is known in Malaysia and the Republic of China. Samples we have compared to Malaysia appear to be the same genus.
The only avenue of hope, since we couldn't get a chemical control for it, was to try to breed for resistance. Because of the devastating effects of the disease, and the seriousness of it, we wanted to try to do it as fast as possible which meant that conventional breeding was excluded. That would take too long. By the time we got something, there would probably be no guava industry left. So we went to the use of tissue culture. We had problems getting guavas into tissue culture. They have a lot of tannins and phenolic compounds so there is a lot of browning in tissue culture. Eventually we used a process of germinating seeds and then we used a fungal filtrate from the fungus where we removed the toxin from the fungus and used that as a selection agent in the tissue culture. Out of the tissue culture selections of 30,000 seedlings, we eventually selected 10 that survived the disease.
This is just a short summary of how we did it: We grew seedlings, put them into tissue culture, treated them with the toxin. Most died. It was a number of processes; we didn't just do it once. We did it a few times through the tissue culture. Those that survived we multiplied in tissue culture and then took them into pot trials. So we had ten selections we took into pot trials. Out of those ten selections, we finally selected three rootstocks which seemed to do well in the pot trials. These were released to a commercial nursery under contract to multiply them. We just called them TS-G-1, TS-G-2 and TS-G-3. TS-G-1 and TS-G-2 are almost resistant. TS-G-3 was tolerant. We found the fungus in the rootstock, but it didn't kill the rootstock. No disease symptoms were observed.
At the same time that they were released to the nursery for propagation for the industry, we put them into field trials. We hadn't done any field trials previously. We used Fan Retief which is our previously normal commercial cultivar as a control, and then we evaluated them. We picked up one problem immediately in the field trial: Fan Retief is resistant to rootknot nematode. We have never had a problem with nematodes in guavas. As soon as we planted resistant rootstocks we started picking up problems. We thought they were dying of the Wilt disease in the field, but when we tested them, we found they were full of rootknot nematodes. The only advantage of that is that one can control nematodes, which we can't do with the fungus disease. We had to then control the nematodes.
We looked at the epidemiology of the disease, but not in depth at this stage, simply because the main thrust of the program was to develop a resistant rootstock. It is thought to spread mainly through the soil. The spores spread around in the air when they are released, but it is thought that they enter the plant through the soil. But there could be some entrance during pruning. Guava pruning is an annual practice; you don't farm guavas without pruning every year.
So, that is how we did our trials in the field. We planted our orchard in soil that was full of the fungus. That orchard is now 4 or 5 years old and none of our rootstocks have died. As long as we control the nematodes (we use chemical control), we are all right. We had no idea of what the fruit quality of the rootstocks was, as they were bred with no consideration for fruit quality. Fan Retief was grafted onto the top. We picked up some problems with that when the scions died back.
Possible reasons for that was that spores were entering wounds on the tree, perhaps caused by pruning. Another possibility is that the commercial nursery that did the propagation originally, did grafting mass production on very young plants so that the graft union was very close to the soil. Some trees I looked at in the field had their graft union under the soil, which defeated the whole purpose of using a resistant rootstock. Or else, they used the TS-G rootstock that was only tolerant so that the fungus was entering the rootstock and moving through to the scion, killing it. Or possibly, the scion material was infected already, before grafting. We never came up with a solution as to what happened. We always recommended, and still do, that the scion material is clean and that graft unions must be at least 30 cm above ground level.
The other alternative was to look at the fruit quality of the rootstocks. If we could get a cultivar that is resistant and has acceptable fruit quality, one doesn't need to graft it. So we looked at fruit characteristics. We again used Fan Retief as a reference cultivar. Our TS-G rootstocks all had acceptable fruit size and colour. If we compared the yields with Fan Retief, the only one that had a significantly lower yield was TS-G-2, which, unfortunately was the rootstock that seemed to have the best resistance. To guava farmers, the main priority is yield. They want so many tons per hectare, otherwise it is not profitable.
TS-G-2 gave us a slightly bigger fruit size as well as a better colour, but they all compared very well with Fan Retief. The quantity of fruit flesh is important in processing. TS-G-1 and TS-G-2 had a broader fruit flesh but the fruit was bigger, so when you took the ratio of the breadth to the fruit flesh, there wasn't any significant difference in any of the cultivars.
Guavas in South Africa are only accepted by the processors if they have a Brix value of about 8. In our breeding program, we always aim for a reading of 10 or higher. TS-G-1 and TS-G-2 both were above 11. TS-G-3 was a little lower than Fan Retief, but not significantly, so they were all acceptable.
In conclusion, TS-G-3 and Fan Retief gave us the highest yields, Fan Retief being the cultivar that is susceptible to the disease. TS-G-2 has significantly lower yield. There is no difference in the fruit breadth. All of the rootstocks had fruit of better colour than Fan Retief, and there was no difference in shelf life, which for guavas is unfortunately short.
None of the rootstocks have ideal fruit quality, but they do compare favourably with the cultivar that was being used. At the moment, new plantings are being made with the rootstock plants being used as cultivars without grafting. The breeding and selection processes are ever continuing using the same methods in the hope of getting better cultivars that are resistant.