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November 2004. A papaya farmer in Samoa has a problem with the yellowing of 1-2-months’ old papaya seedlings after transplanting to the field. What could be the cause? It was said the drainage is not good. The seedlings are less than a metre high, with the bottom 1-3 leaves yellow and the rest still green. When the wilted plants were dug up, the roots were found to be rooten. The land is waterlogged, and there are also fau (Hibiscus tiliaceus) trees surrounding the area. These trees have been associated with Pythium rot on taro in Samoa.
The same condition was also reported in Fiji where there are shallow soils over soapstone, compared to soils over limestone.
From the description of yellowing and quick death of seedling, the most probable reason seems to be root rot, caused by Phytophthora, Pythium or Fusarium species. Examination of a few roots of recently dead seedlings will enable confirmation.
North Queensland has experienced heavy losses when planting seedlings into wet weather in summer. So, to reduce the incidence of the problem, either plant coming into the dry/cooler season or plant seeds and thin out later. There are fewer losses with direct planting. Hilling up is useful. Also, avoid filling the gaps immediately, but dig up the planting holes, and expose the soil to the sun, allowing it to dry up completely. Well drained soil and good field sanitation are important to avoid recurrence.
The experience in Darwin, Northern Territory is somewhat different: There the wilt does not appear to be associated with a fungus. Papaya growing near concrete thrive in contrast to others that yellow and dieback in the absence of concrete. In other words, this condition responds to calcium. This is a physiological disease.
To differentiate between the drainage suggestions and calcium deficiency, it was recommeneded to do the following:
Drainage: Construct drainage canals between rows. Made the drain one foot outside the drip circle.
Calcium: Apply lime or gypsum around the plants. As Ca does not easily move in the soil, it should be brought as close as possible to the root system, without damaging the roots. Place it in areas where roots will grow towards it. If the problem is on inland volcanic soil, then it’s unlikely to be a nutrient problem. If its on the coast on coral rubble then nutrients, particularly iron deficiency, could be an issue.
Work through the options: Look at other plant species in the vicinity. Do they have nutrient deficiency symptoms? Are there any pawpaw growing wild there? Are they healthy? Did all the plants go yellow at once or did they drop one by one? If they all went wrong at the same time then it probably is not a root rot as root problems tend to be more ‘hit and miss’ in the block. Did the grower apply any weedicides in the area? For example, Roundup drift onto young pawpaw plant can be very damaging. If there is no obvious solution from that angle, isolate from the roots onto selective medium to see if Phytophthora or Pythium is there. Use the CARP selective medium. The most common Phytophthora on pawpaw in the Pacific is P palmivora in roots and as a white rot on the fruit and stems of older plants.
There was also mention of the potentail of bioagents, Trichoderma harzanium, and T viride.
Phytoplasma disease occurs in papaya in Australia. It was not thought likely that the wilted plants in Samoa were suffering from this disease, but it is worthwhile considering it, if only to eliminate the possibility. The phytoplasma thought to be causing papaya dieback belongs to a group currently called the Stolbur group (16SrXII); it is a serious disease in both Australia and New Zealand.
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