Potato virus y (PVY) in Irish potatoes (Solanum tuberosum) and tree tomato (Cyphomandra betaceae) and the influence of potato susceptibility to the virus and the aphid virus-vector on the spread of PVY Mosaic in Kenya

Abstract

Potato virus Y (PVY) occurs on some important crops, weeds and shrubs all over the world. The most common known hosts of PVY strains are Solatium tuberosum, Capsicum annuum, Nicotianna tabacum, Lycopersicon esculentum among crops, and Solatium atropurpureum and Datura metel among weeds. PVY has been also reported as a potent pathogen of Cyphomandra betaceae, a fruit shrub commonly known as Tree-tomato, largely distributed in small scale farm plots all over East Africa, with its habitat stretching into Kenya, Tanzania, Uganda, Rwanda, Burundi and eastern highland of Zaire. The natural habitat of this fruit shrub is mostly restricted to highlands of these countries where the irish potato is one of the most important staple food and cash crop for the majority of farmers. However, tree-tomato fruit is mostly consumed by the household and rarely considered as a source of income. In all these areas there is not a single tree without a virus disease like symptoms. As PVY has been reported elsewhere as being virulent to this fruit shrub, the first part of this work was aimed at ascertaining the identity of the causal agent of the endemic virus disease like symptoms observed in eastern Africa. The hypothesis was that if the causal agent is of a viral nature, then the most probable agent may be one the three viruses that have been reported on tree-tomato, namely Cucumber mosaic virus (CMV), Potato virus X (PVX) or PVY. The second part of this study dealt with the form of occurrence of PVY strains on potato variety in two selected areas in Kenya, where the irish potato is considered as one of the most important crops. The third and last part of this study examined the influence of the susceptibility of mostly grown potato varieties to aphid and to PVY on the spread of PVY infection in the field.

The causal agent of tree-tomato diseases was identified through host range, physical properties, electron microscopy and transmission tests. The host range showed that only Solanaceae species were susceptible. Other families such as Chenopodiaceae, Leguminosae, Cucurbitaceae, Amaranthaceae and Compositae were not susceptible. The fact that none of the Cucurbitaceae plant species was susceptible confirms that the virus prevailing in tree-tomato in Kenya is not a CMV strain. In addition, electron microscopic observation showed that the virus had flexuous filamentous particles and not polyhedral particles as CMV. Observation of tested indicator plants, the physical properties, the modal length of 738 nm in a purified preparation, and the ELISA have proved that this virus is a strain of PVY readily transmissible to potato and pepper crops. The form of occurrence of PVY strains in the two selected areas of potato production was carried out by the observation of symptoms induced by various isolates on a set of indicator plant species and varieties and by ELISA using antisera against PVY, PVX, PVS and PVA. The various symptoms observed on differential hosts showed clear deviation from PVY strains. The ELISA showed that PVY occurs rarely as single infection but mostly within a complex of two, three or four viruses. The results of ELISA tests showed that when collecting infected potato plants samples in the field, around 85.7% of the samples contain potato virus Y. However, only 16.7% of the sample were pure PVY infection. These results show that PVY can be found at the rate of 69% in the form of mixed infection with one, two or three other viruses. Association with one virus (X or A) was estimated at 25.5%, that with two different viruses at 32.5%, and that with all three other viruses at 9.5%.

Results showed that among 13 PVY isolates could be separated into two groups. The common strains PVY°, not causing veinal necrosis on tobacco varieties and the necrotic strains PVYN, causing necrotic symptoms on Nicotiana tabacum var ’White Burley’, ’Samsun’ and ’Kentucky’. In addition, within each group, strains showed differences in their ability to infect pepper cv ’Long Red Cayenne’. The study showed an equal proportion of common strains PVY° (7 strains) and necrotic strains PVYN (6 strains). It equally showed that most tested strains are virulent on pepper (9/13). Thus, when not grown for commercial purposes, pepper should be considered as potential reservoir host for most Kenya PVY strains. The susceptibility level to PVY infection also showed a wide variation among varieties (ratio of 7:1 for the 1st crop and 3:1 in the second crop). Such variation should mainly be attributed to several factors, among which the genetic resistance to the virus itself, the resistance to vectors, the initial infection rate in potato tuber seeds of each variety, and the transmission potential of the most predominant vector. The mechanism of resistance or susceptibility to PVY has been described by several authors. However, the comparison of data on PVY rate and aphid infestation rate showed that there is a strong correlation between the two parameters. The first crop shows that there is a weak correlation (r2 = 0.25). The same fact was observed in the second trial with a more significant correlation (r2 = 0.72). However, the behaviour of ’Kenya Baraka’ (lowest aphid population and highest rate of PVY) suggests that a higher susceptibility to PVY cannot be corrected by the inclusion of aphid-resistant gene into it. This means a variety should have a combination of both types of resistance to give satisfactory performance. The other important parameter to consider is the effect of PVY infection on tuber yield. It appears that the rule that higher virus infection means a reduction

on yield does not apply with a uniform intensity on a set of varieties. This means that two varieties may have the same rate of virus infection but be affected differently on their yield. Another parameter to consider here is the genetic production potential of a variety. Among the eight varieties tested, some are classified as high, medium or low-yielding. From that assumption, a mere comparison of yields cannot be a sufficient criteria to assess the pressure of virus infection on the variety yield. Among the eight varieties, ’Annet’, ’Kenya Baraka’, ’Feldeslohn’, ’Dutch Robjin’ and ’Bvumbe’ are considered as high producers and can yield over 40 tonnes per ha, while ’Kerr’s Pink’, ’Pimpernell’ and 'B.53' are considered as medium and yield between 20 to 40 tonnes per hectare. The obtained production shows that varieties with the highest PVY rate during the second crop, namely ’Kenya Baraka’, ’Dutch Robjin’ and ’Feldeslohn’ have recorded the highest yield reduction varying from 47.8 to 52.5%, while varieties with the lower PVY rate, notably ’Pimpernell’ and ’Annet’ have a lower yield reduction varying between 22.3 - 23.9 %. The intermediate level shows the same trend, this group includes ’Bvumbe’ and ’B.53’ with a reduction of 33.8 to 35.7%, while the variety 'Kerr’s Pink’ is in a group of its own where we find the highest PVY rate (52%) corresponding to a comparatively low reduction (29.6%) in yield as compared to the highest and the average group.The conclusion is that reduction should be considered as a more reliable parameter to assess than the yield itself.